Nerve Conduction Studies: Principles, Methodology, Interpretation, and Clinical Applications

Introduction

Physiological Basis

• Motor NCS: The nerve is stimulated while recording from a target muscle; the resulting potential is the compound muscle action potential (CMAP), also called the M wave.
• Sensory NCS: A sensory or mixed nerve is stimulated and the resulting sensory nerve action potential (SNAP) is recorded from another point along the same nerve, either orthodromically (in the physiological direction) or antidromically (opposite direction). Antidromic sensory studies offer technical advantages and are more widely used in routine practice.
• Mixed NCS: Both sensory and motor fibers are stimulated and recorded from a mixed nerve, yielding the mixed nerve action potential (MNAP).

Equipment and Technical Procedure

Stimulators

Recording Electrodes

Parameters Measured and Their Significance

1. Conduction Velocity

• Normal motor conduction velocities: 50-70 m/s in the upper limbs, 40-60 m/s in the lower limbs.
• Velocities in normal subjects range from approximately 40-45 m/s (minimum) to 65-75 m/s (maximum) depending on the nerve studied (Adams and Victor's Principles of Neurology, 12th Edition).
• Slowing of conduction velocity is the hallmark of demyelinating pathology.

2. Distal Latency

3. Amplitude

• CMAP amplitude is measured in millivolts (mV) and reflects the summated electrical potential of all activated muscle fibers within the recording electrode's pickup region. It is a semiquantitative index of the number of functioning motor axons and the innervated muscle volume.
• SNAP amplitude is measured in microvolts (μV) and reflects the number of functioning large sensory axons.
• Reduction in amplitude is the primary electrodiagnostic signature of axonal loss (axonopathy), regardless of the etiology.

4. Waveform Duration and Morphology

Late Responses: F Waves and H Reflexes

• F wave: A small, late motor response produced by antidromic activation of anterior horn cells, followed by orthodromic transmission back to the muscle. It assesses the entire motor axon from ventral horn to muscle, making it useful for detecting radiculopathies and proximal demyelination.
• H reflex: The electrical equivalent of the ankle jerk, predominantly assessing the S1 arc via the tibial nerve and soleus. Absent or asymmetric H reflex strongly suggests S1 radiculopathy or proximal neuropathy.

Physiological Variables and Sources of Error

Temperature

Age

Height and Nerve Length

Anatomical Anomalies

Electrodiagnostic Interpretation: Axonopathy vs. Demyelination

Clinical Applications

1. Polyneuropathy

2. Entrapment Neuropathies

3. Radiculopathy and Plexopathy

4. Neuromuscular Junction Disorders

5. Motor Neuron Disease

6. Nerve Injury Assessment

Limitations and Complementary Modalities

Conclusion

• Bradley and Daroff's Neurology in Clinical Practice (Chapter on Nerve Conduction Studies and EMG)
• Adams and Victor's Principles of Neurology, 12th Edition (Chapter 2: Electromyography and Nerve Conduction Studies)
• Goldman-Cecil Medicine, International Edition (Chapter 366: Neurologic Diagnostic Tests)
• Pfenninger and Fowler's Procedures for Primary Care, 3rd Edition (Chapter 224: NCS Interpretation)
• Recent literature: Tankisi H et al., Clin Neurophysiol 2024 [PMID: 38805900] (muscle excitability testing as a complementary modality)

Tendon Suturing Techniques

Introduction

Tendon Biology and Healing

• Intrinsic healing: Directed by tendon fibroblasts (tenocytes) within the tendon substance. This is the dominant mode and produces healing with minimal adhesion.
• Extrinsic healing: Mediated by fibroblasts invading from the surrounding tendon sheath and peritendinous tissue. It is a secondary contributor but is a major source of adhesion formation, which restricts gliding and impairs function.

Characteristics of an Ideal Tendon Repair

1. Allows easy placement of sutures in the tendon
2. Achieves secure suture knots
3. Creates a smooth juncture of tendon ends
4. Produces minimal gapping at the repair site
5. Minimally interferes with tendon vascularity
6. Provides sufficient strength throughout healing to permit early motion stress

Suture Material

• Monofilament nylon (Ethilon): easier to handle than steel, but permits earlier gap formation at the repair site compared with braided materials.
• Polypropylene (Prolene): similar to nylon; loses strength faster than caprolactam-based sutures.
• Polydioxanone (PDS): a slowly absorbable monofilament comparable in strength to polypropylene over 28 days; useful when permanent suture is undesirable.
• Braided polyester (Ticron, Mersilene, FiberWire): the most widely used core suture in current practice. Braided polyester provides superior resistance to disrupting forces and gap formation, handles well, and has reliable knot characteristics.
• Braided polyethylene: biomechanically the strongest non-metallic suture, though less widely used due to cost.

The Core Suture: Principles and Strand Count

• Two-strand repairs (e.g., simple modified Kessler alone) are no longer considered adequate for early active mobilisation.
• Four-strand repairs represent the minimum for early active motion protocols.
• Six- and eight-strand repairs are now preferred in zone II and in all zones where early active flexion is planned (IFSSH 2026 consensus; Miller's Review of Orthopaedics, 9th Edition).

• Purchase (the longitudinal distance from the cut tendon end to the transverse component of the core suture): should be 0.7-1.2 cm. Longer purchase increases holding strength.
• Locking vs. grasping configuration: Locking-loop sutures trap bundles of tendon fibers and resist gap formation more effectively than simple grasping configurations.
• Knot position: Knots placed between the tendon stumps are foreign bodies that impede gliding and should be avoided; the 2026 IFSSH guidelines explicitly recommend against inter-stump knot placement.
• Dorsal vs. volar placement: Dorsally placed core sutures provide approximately 58% greater strength. However, traditional teaching placed sutures in the volar third to avoid compromising intratendinous blood supply. Current practice recognises that with multi-strand repairs, some dorsal placement is acceptable.

Core Suture Configurations

1. Bunnell Stitch

2. Kessler Stitch

3. Modified Kessler (Tajima) Stitch

4. Tsuge (Loop) Stitch

5. Savage Six-Strand Repair

6. Lee Four-Strand Technique

7. Indiana Four-Strand Repair with Running Lock Suture

8. M-Tang Repair (Multiple-Looped Suture)

9. Six-Strand Double-Loop Repair (Lim-Tsai)

10. Cruciate and Four-Strand Cruciate Repairs

The Peripheral (Epitendinous) Suture

• Reduces bulk at the repair site, minimising the risk of triggering at pulley level
• Smooths tendon contour, reducing resistance to gliding
• Enhances overall repair strength by 10-50% - a widely cited figure from biomechanical studies
• Supports approximately 50% of the load to failure in some configurations, making it not merely a cosmetic addition but a structural component

Flexor Tendon Zones and Repair Strategy

• Four-strand core sutures with locking components and buried knots are preferred as a minimum.
• The A2 and A4 pulleys must be preserved to prevent bowstringing (A2 is the most important).
• A portion of A2 may be incised (but not entirely excised) to facilitate tendon passage and reduce triggering at the repair site.

Extensor Tendon Repair

• Zone I-II (over DIP and PIP joints): These injuries (mallet finger, central slip avulsion) are frequently treated conservatively with splinting. When operative repair is needed, a modified Bunnell or Kleinert modification of the Bunnell suture is stronger than figure-of-eight or mattress sutures for core repair. A Silfverskiöld continuous suture reinforcement may be added.
• Zone III-IV (over PIP joint and proximal phalanx): Both the central slip and lateral bands require attention; boutonnière deformity results from untreated central slip injuries.
• Zone V-VIII (over MCP joint to forearm)**: The extensor environment is more forgiving given wider cross-sectional area and less confined anatomy. Modified Bunnell, modified Kessler, or augmented Becker configurations with a cross-stitch reinforcement provide strong repairs in zones IV-V. For zones VI-VIII, modified Kessler, modified Bunnell, modified Becker (MGH), and Krackow-Thomas techniques are all appropriate.

Suture Material for Core vs. Epitendinous Layers: Summary

Postoperative Rehabilitation

• A minimum four-strand repair with epitendinous suture is required for early active flexion protocols. Immobilisation-based protocols are no longer recommended for compliant adult patients (Miller's Review of Orthopaedics, 9th Ed; IFSSH 2026).
• The WALANT technique (Wide Awake Local Anaesthesia No Tourniquet, using lidocaine with epinephrine) allows intraoperative assessment of repair integrity with active patient participation, and is increasingly adopted as best practice.
• Place-and-hold exercises (passive flexion followed by active holding) are now recommended to be abandoned in favour of true early active flexion (IFSSH 2026).
• Young children who cannot comply with protected motion protocols require cast immobilisation for four weeks.

Complications

• Rupture: Risk is greatest at three weeks; failure occurs at the knots. Inadequate strand count, excessive tension during early mobilisation, and knot placed between stumps are the main technical causes.
• Adhesion formation: The principal obstacle to functional recovery, especially in zone II. Minimised by atraumatic handling, meticulous technique, smooth repair site, and early active mobilisation.
• Triggering: Caused by a bulky repair impinging on the flexor sheath at pulley level. Prevented by well-placed epitendinous suture and, if necessary, partial A2 excision.
• Bowstringing: Results from sacrifice of the A2 pulley; must be avoided.

Conclusion

• Campbell's Operative Orthopaedics, 15th Edition, 2026 (Chapter 71: Basic Tendon Repair Techniques and Flexor Tendon Injuries)
• Miller's Review of Orthopaedics, 9th Edition (Chapter on Tendon Injuries and Overuse Syndromes)
• Tang JB, Lalonde D, Fernandes CH et al. "The IFSSH consensus and current guidelines on flexor tendon repairs and reconstruction." J Hand Surg Eur Vol, 2026 [PMID: 41637469]
• Tang JB, Pan ZJ, Munz G. "Flexor Tendon Repair Techniques: M-Tang Repair." Hand Clin 2023 [PMID: 37080646]

Core Suture Configurations

1. Bunnell Stitch

The classic Bunnell stitch - a crisscross zigzag pattern passing through the tendon substance. Note the extensive intratendinous passage, which compromises vascularity. Largely abandoned for primary repair.

2. Kessler Grasping Stitch (2-strand)

The original Kessler stitch. The suture passes transversely across each tendon end, creating a grasping purchase at two points, with the knot at the repair site. This is the two-strand form.

3. Modified Kessler (Tajima) Stitch - Single Knot

The most widely used two-strand core repair. The knot is placed at the repair site. Two of these placed at 90° to each other yield a four-strand repair - the minimum recommended for early active motion.

4. Tsuge (Loop) Stitch

Uses a pre-formed looped suture. The loop passes through the tendon and the free ends are knotted, creating a grasping purchase with reduced knot bulk. The double Tsuge modification creates 6 strands.

5. Double Grasping (One Suture)

A locking-loop variant where one suture achieves double grasping purchase on each tendon end, improving resistance to gap formation compared with a simple grasping stitch.

6. Crisscross Stitch

A four-strand crisscross configuration (Figure 71.5A from Campbell's). More intratendinous passage than Kessler variants; note the knot exits at the repair surface.

7. Mason-Allen (Chicago) Stitch

The Mason-Allen (Chicago) stitch combines a transverse horizontal mattress component with longitudinal bites, providing secure locking purchase. Widely used for larger tendons (e.g., rotator cuff, Achilles).

8. Savage Six-Strand Repair

The Savage repair with six numbered strands crossing the repair site in a crossing pattern (loops 1-6 as labelled). One of the strongest described configurations; technically demanding but resists gap formation effectively.

9. Lee Four-Strand Technique

The Lee technique achieves four strands using two sutures, with two knots buried within the repair site. This diagram illustrates the suture path and cross-section orientation through the tendon.

10. Core + Epitendinous Repair (combined view)

Summary Table

Trendelenburg Gait

Introduction

Normal Hip Biomechanics During Gait

Anatomy of the Hip Abductor Mechanism

• Gluteus medius: The principal abductor. It arises from the outer surface of the ilium between the anterior and posterior gluteal lines and inserts on the superolateral surface of the greater trochanter. Its anterior fibres assist internal rotation; its posterior fibres assist external rotation.
• Gluteus minimus: Lies deep to gluteus medius, arising from the outer ilium between the anterior and inferior gluteal lines, inserting on the anterior aspect of the greater trochanter. It also assists internal rotation.
• Tensor fascia lata (TFL): Acts as a minor abductor through its insertion into the iliotibial band.

Figure 7.2 from Campbell's Operative Orthopaedics (15th Ed, 2026): Trendelenburg sign is positive when the affected hemipelvis sags during one-legged stance.

The Trendelenburg Sign: Clinical Testing

1. The examiner stands behind the patient.
2. The patient is asked to stand on one leg (the affected limb).
3. Normally, the contralateral (unsupported) hemipelvis rises or remains level.
4. A positive test is recorded when the unsupported hemipelvis drops, indicating failure of the stance-limb abductors to maintain pelvic level.

The Trendelenburg Gait: Mechanism and Appearance

1. As weight is borne on the affected side, the weakened abductors fail to prevent the contralateral hemipelvis from dropping.
2. To compensate and maintain the centre of gravity over the weight-bearing limb (and prevent an uncontrolled fall to the contralateral side), the patient lurches the trunk laterally toward the affected side - the "abductor lurch" or Trendelenburg lurch.
3. This lateral trunk shift brings the body's centre of gravity directly over the stance hip, reducing the moment arm of body weight about the hip joint and thereby reducing the demand on the abductors.
4. When both hips are affected, the lurch alternates from side to side with each step, producing the characteristic waddling gait (Adams and Victor's Principles of Neurology, 12th Ed).

Trendelenburg Gait vs. Antalgic Gait: Key Distinction

Causes of Trendelenburg Gait

1. Neurological Causes

Superior Gluteal Nerve Injury

• Hip arthroplasty via lateral/anterolateral (Hardinge/Watson-Jones) approaches: Splitting the gluteus medius more than 5 cm proximal to the greater trochanter violates the "safe zone" and risks superior gluteal nerve damage. Vigorous acetabular retraction and extreme leg positioning during femoral preparation are further risk factors (Campbell's Operative Orthopaedics, 15th Ed, 2026).
• Pelvic fractures: Particularly those that extend into the greater sciatic foramen.
• Space-occupying pelvic lesions: Tumours or haematomas compressing the nerve within the pelvis.
• Intramedullary femoral nailing: A significant decrease in hip abductor strength has been noted following this procedure.

Nerve Root Lesions

• L4-L5 radiculopathy: The superior gluteal nerve is derived from L4, L5, and S1. A lateral disc herniation at L4-5 or L5-S1 can produce abductor weakness and Trendelenburg gait. The classic board question involves a left-sided Trendelenburg gait and L4-5 pathology.
• Lumbosacral plexopathy.

Neuromuscular Diseases

• Progressive muscular dystrophies (especially Duchenne and Becker): Proximal muscle weakness prominently affects the hip girdle, producing the classic waddling gait.
• Spinal muscular atrophy (SMA): Chronic forms produce proximal lower limb weakness.
• Inflammatory myopathies (polymyositis, dermatomyositis).
• Poliomyelitis: Historically a common cause; still relevant in endemic regions and in post-polio syndrome.
• Myelomeningocele and other spinal cord lesions.

2. Structural Hip Causes

Developmental Dysplasia of the Hip (DDH)

Coxa Vara

Perthes Disease (Legg-Calvé-Perthes Disease)

Slipped Capital Femoral Epiphysis (SCFE)

Osteoarthritis of the Hip

Greater Trochanteric Fractures

Gluteus Medius/Minimus Tears

3. Post-Arthroplasty Abductor Insufficiency

• Superior gluteal nerve injury (see above)
• Direct iatrogenic injury to the gluteus medius during surgical approach (particularly lateral/anterolateral approaches)
• Trochanteric escape or non-union after trochanteric osteotomy
• Adverse local tissue reactions (ALTR) to metal-on-metal or other prosthetic bearings
• Osteolysis due to bearing wear or infection destroying the abductor insertion

Clinical Assessment of the Patient with Trendelenburg Gait

History

• Onset (sudden vs. insidious), duration, progression
• Prior hip surgery and approach used
• Hip pain (suggests structural cause)
• Back pain and radicular symptoms (suggests neurological cause)
• Developmental history (DDH, Perthes)
• Family history of neuromuscular disease
• History of trauma

Observation of Gait

• Lateral trunk lurch direction and timing
• Pelvic drop during stance phase
• Whether gait is antalgic, Trendelenburg, or combined
• Symmetry (unilateral vs. bilateral)
• Associated flexion contracture (compensatory lumbar hyperlordosis in stance)
• Snapping iliotibial band

Trendelenburg Test

Neurological Examination

Leg Length Assessment

Imaging

• Plain radiographs (AP pelvis and lateral hip): assess for osteoarthritis, DDH (sourcil, CE angle, Shenton's line), coxa vara, SCFE, greater trochanter fracture/malunion, previous arthroplasty components
• MRI: gold standard for gluteus medius/minimus tears, ALTR, and nerve assessment
• Nerve conduction studies and EMG: to localise and characterise neurological lesions

Management Principles

• Neurological recovery (e.g., superior gluteal nerve palsy post-THA): expectant management; most recover within 6-18 months if cause is neuropraxia. Physiotherapy for abductor strengthening once reinnervation begins.
• Gluteus medius/minimus tears: physiotherapy-led abductor strengthening for partial tears; surgical repair (transosseous or anchor-based) for full-thickness tears causing significant disability.
• Post-THA abductor insufficiency: surgical reconstruction using tendon transfer (TFL, Achilles allograft augmentation) or repair with anchor fixation for chronic tears; revision of components if ALTR or osteolysis is the cause.
• DDH, Perthes, coxa vara: appropriate reconstructive surgery (pelvic osteotomy, femoral osteotomy) to restore normal biomechanics.
• Radiculopathy: treat the underlying spinal pathology.
• Neuromuscular disease: supportive management, orthotics (hip-knee-ankle-foot orthoses in severe cases), and multidisciplinary rehabilitation.

Clinical Pearls for the Examination

1. The Trendelenburg gait is caused by abductor weakness on the stance side - the trunk lurches toward the weak side (confusingly, the side of the lesion).
2. In the antalgic gait, the patient also leans toward the painful side - but to reduce joint reaction force by shortening the abductor lever arm, not because the abductors are weak. The key difference is the shortened stance phase in antalgic gait.
3. A positive Trendelenburg test = pelvis drops away from the stance limb = weakness of the stance limb abductors.
4. In bilateral hip disease (DDH, muscular dystrophy, bilateral OA), the alternating bilateral Trendelenburg produces the waddling gait.
5. The safe zone for splitting gluteus medius during hip approach is 5 cm proximal to the greater trochanter - beyond this risks superior gluteal nerve injury and postoperative Trendelenburg gait.
6. Coxa vara produces Trendelenburg gait with intact muscle - purely due to altered biomechanics (reduced abductor lever arm).
7. In L5 radiculopathy, Trendelenburg gait may coexist with foot drop and dermatomal sensory loss - the gait pattern alone does not distinguish hip from spinal pathology.

Conclusion

• Campbell's Operative Orthopaedics, 15th Edition, 2026 (Chapters 4 and 7: Hip Examination and Neurological Injuries in THA)
• Adams and Victor's Principles of Neurology, 12th Edition (Chapter on Waddling/Gluteal Gait)
• Gray's Anatomy for Students (Trendelenburg's Sign)
• Bailey and Love's Short Practice of Surgery, 28th Edition (Hip Biomechanics and Examination)
• Firestein & Kelley's Textbook of Rheumatology (Hip Physical Examination)
• Miller's Review of Orthopaedics, 9th Edition
• Harrison's Principles of Internal Medicine, 22nd Edition (2025)
• Pearce AN et al. "Diagnosis and Treatment Options of Abductor Insufficiency After Total Hip Replacement." Orthop Clin North Am 2022 [PMID: 35725034]
• Mashabi A et al. "Gait Compensation among Children with Non-Operative Legg-Calvé-Perthes Disease." Healthcare 2024 [PMID: 38727452]

Patellar Tendon Bearing Prosthesis

Introduction

The Transtibial Residual Limb: Surgical Prerequisites

Residual Limb Length

• An adequate lever arm to drive the prosthesis through space
• Sufficient soft tissue coverage over the distal bone end
• Clearance for prosthetic components (ankle-foot unit) distal to the socket

Skin Flaps

Muscle Balancing

Bony Prominences

Weight Bearing Principles

Figure 21-1 from Rockwood and Green's Fractures in Adults (10th Ed, 2025): Red arrows = hydrostatic force transmission (PTB principle); Blue arrow = direct end weight bearing (knee disarticulation principle). Panel B shows the transtibial PTB socket with hydrostatic force distribution.

Anatomy of Pressure Distribution: Tolerant vs. Intolerant Areas

Pressure-Tolerant (Weight-Bearing) Areas

1. Patellar tendon - the primary load-bearing zone, against which the anterior socket brim presses during stance phase (akin to patellar tendon-bearing cast application)
2. Medial tibial flare (proximal medial tibial metaphysis) - broad cortical surface amenable to pressure
3. Anterior compartment musculature - soft tissue cushion over the anterior tibia
4. Gastrocnemius muscle (posterior stump) - well-vascularised, durable soft tissue
5. Fibular shaft (mid and proximal portions)

Pressure-Intolerant (Relief) Areas

1. Tibial crest - subcutaneous, sharp bony ridge; most vulnerable to pressure necrosis
2. Tibial tubercle - bony prominence directly underlying the patellar tendon insertion
3. Distal fibula and fibular head - subcutaneous; fibular head is also vulnerable to peroneal nerve compression
4. Common peroneal nerve - as it winds around the fibular head; compression causes peroneal palsy
5. Hamstring tendons (semitendinosus, biceps femoris) - posteriorly, where they cross the knee
6. Distal stump end - even with well-padded closure, direct end-bearing is not intended and can cause ulceration

The PTB Socket: Design and Variants

Classic PTB Socket (Radcliffe-Foort, 1959)

• Has a patellar tendon bar - an anterior brim projection that directly loads the patellar tendon
• Provides relief channels over pressure-intolerant areas (tibial crest, fibular head)
• Is moulded with the knee in slight flexion (5-10°) to facilitate patellar tendon loading
• Uses a soft insert (polyethylene foam or more recently silicone/thermoplastic elastomer) as a liner between the residual limb and rigid outer shell
• Achieves suspension via a supracondylar strap or cuff

Supracondylar (SC) Socket (PTB-SC)

• The residual limb is shorter than 5 cm (limited lever arm)
• Suspension reliability is a priority (e.g., high-activity patients)
• An external cuff is cosmetically unacceptable

Supracondylar-Suprapatellar (SC-SP) Socket (PTB-SC-SP)

• Provides the most secure bony suspension of all PTB variants
• Offers excellent mediolateral stability, so no additional straps or cuffs are required
• Is indicated for very short stumps, patients with ligamentous knee instability, and those requiring maximum suspension
• The enclosed patella may restrict some patients; cosmetically more bulky

Total Surface Weight Bearing (TSWB) Socket

Double-Walled Socket Construction

• Flexible inner socket (thermoplastic, silicone, or TPE) - provides total contact, accommodates volume changes, and protects skin
• Rigid outer shell (carbon fibre, fibreglass, or rigid thermoplastic) - provides structural support and connects to the prosthetic shank and foot

Suspension Systems

1. Supracondylar Cuff (Strap)

2. Supracondylar Bony Lock (SC and SC-SP sockets)

3. Suction Suspension

4. Elevated Vacuum Suspension

5. Sleeve Suspension

6. Corset (Thigh Corset) Suspension

• Knee instability requires lateral support
• Very short stumps provide insufficient purchase for other methods
• Dysvascular patients with fragile skin who cannot tolerate suction
• Resource-limited settings

Prosthetic Components Distal to the Socket

Shank (Pylon)

• Endoskeletal (modular): load-bearing central pylon covered by a soft cosmetic foam exterior - the most common modern design. Allows easy adjustment of alignment.
• Exoskeletal (crustacean): hard load-bearing outer shell. More durable but cannot be adjusted easily.

Prosthetic Foot

• Seattle Foot
• Carbon Copy II/III
• Flex Foot (with posterior projection for heel strike and split-toe configuration)
• Ceterus Foot (with integrated shock absorber)

Alignment

Bench Alignment

• The socket is flexed 5-10° to facilitate patellar tendon loading and prevent socket brim impingement in the popliteal fossa
• The foot is positioned such that the ground reaction force (GRF) passes approximately through the knee joint centre in the sagittal plane during mid-stance

Dynamic Alignment

• Lateral trunk shift (may indicate socket varus/valgus malalignment)
• Knee instability (excessive socket flexion, foot plantar flexion)
• Whip (rotational malalignment at the foot-pylon interface)
• Uneven step length (limb length discrepancy)

Special Considerations

The PTB Cast

Paediatric Considerations

Vascular Amputees

• Skin fragility demands meticulous socket fit without pressure point creation
• Total contact is essential to prevent dependent oedema
• Wound care must be integrated into the rehabilitation pathway
• Silicone liners are preferred over hard direct-contact sockets
• Below-knee amputation retains the knee joint, dramatically improving rehabilitation potential and energy expenditure compared with transfemoral amputation

Rehabilitation and Functional Outcomes

1. Pre-operative counselling and goal setting (ideally involving the prosthetist)
2. Immediate post-operative phase: rigid dressing or pneumatic post-amputation mobility (PPAM) aid; prevention of flexion contracture at the knee (position in extension)
3. Stump maturation: residual limb bandaging in a specific figure-of-eight pattern to shape the stump; volume stabilisation typically takes 6-18 months
4. Temporary prosthesis: fitted once wound healed and stump volume stabilising
5. Definitive prosthesis: fitted when stump volume is stable; socket replaced as volume changes
6. Gait training: initiated with parallel bars, progressing to walking aids, then independent ambulation
7. Advanced activities: as dictated by K-level functional classification

Complications of the PTB Prosthesis

Conclusion

• Campbell's Operative Orthopaedics, 15th Edition, 2026 (Chapter 31: Amputations)
• Miller's Review of Orthopaedics, 9th Edition (Chapter 10: Prosthetics)
• Rockwood and Green's Fractures in Adults, 10th Edition, 2025 (Chapter 21: Amputations; Chapter 4: Patellar Tendon-Bearing Cast)
• Pye's Surgical Handicraft, 22nd Edition (Chapter 25: Amputations)
• Pfenninger and Fowler's Procedures for Primary Care, 3rd Edition (PTB Cast Application)

Patellar Tendon Bearing Prosthesis

Introduction

The Transtibial Residual Limb: Surgical Prerequisites

Residual Limb Length

Skin Flaps

Muscle Balancing

Neurovascular Management

Principle of Weight Transfer

Anatomy of Pressure Distribution: Tolerant vs. Intolerant Areas

Pressure-Tolerant (Weight-Bearing) Areas

1. Patellar tendon - the primary load-bearing zone; the anterior socket brim presses directly against it during stance
2. Medial tibial flare - broad, cortical surface of the proximal medial tibial metaphysis
3. Anterior compartment musculature - soft tissue cushion over the anterior tibia
4. Gastrocnemius muscle - posteriorly, well-vascularised and durable
5. Fibular shaft (mid and proximal portions)

Pressure-Intolerant (Relief) Areas

1. Tibial crest - subcutaneous, sharp; most vulnerable to pressure necrosis
2. Tibial tubercle - prominent insertion of the patellar tendon; paradoxically relieved despite proximity to the loading zone
3. Distal fibula and fibular head - subcutaneous prominences
4. Common peroneal nerve - as it winds around the fibular neck; compression causes peroneal palsy
5. Hamstring tendons - posteriorly at the popliteal crease
6. Distal stump end - direct end-bearing is not intended and causes skin ulceration

From Rockwood and Green's (2025): Panel B shows the transtibial PTB socket with predominant hydrostatic force transmission (blue arrow = ground reaction force through residual tibia; red arrows = socket interface pressure distribution).

PTB Socket: Design and Variants

1. Classic PTB Socket (Radcliffe-Foort, 1959)

• Patellar tendon bar - anterior brim projection that directly loads the patellar tendon
• Relief channels over pressure-intolerant areas (tibial crest, fibular head, peroneal nerve)
• Soft insert liner between stump and rigid shell
• Suspension via a supracondylar cuff/strap
• Knee positioned in 5-10° flexion during fabrication

2. Supracondylar (PTB-SC) Socket

• Residual limb shorter than 5 cm (limited lever arm)
• High-activity patients requiring reliable suspension
• Patients who find the cuff cosmetically unacceptable
• Added mediolateral knee stability required

3. Supracondylar-Suprapatellar (PTB-SC-SP) Socket

• Maximum bony suspension - no straps or cuffs required
• Excellent mediolateral stability
• Best suited to very short stumps and knees with ligamentous laxity

4. Total Surface Weight Bearing (TSWB) Socket

5. Double-Walled Socket Construction

• Flexible inner socket (thermoplastic/silicone/TPE) - total contact, protects skin, accommodates volume changes
• Rigid outer shell (carbon fibre, fibreglass, rigid thermoplastic) - structural support, connects to distal components

Suspension Systems

Prosthetic Components Distal to the Socket

Shank (Pylon)

• Endoskeletal (modular): load-bearing central pylon with soft cosmetic foam exterior - the dominant modern design; allows easy alignment adjustment
• Exoskeletal: hard outer load-bearing shell; more durable but non-adjustable

Prosthetic Feet

• Non-articulated; rigid keel with cushioned heel wedge
• Simulates plantar flexion at heel strike via heel compression
• Standard for K1 (household) ambulators
• Advantages: low cost, durable, low maintenance
• Limitation: no energy return; poor terrain accommodation

• Hinge allows dorsiflexion/plantar flexion
• Aids knee stability at heel strike in weak quadriceps
• K1 patients with stability concerns; poor durability limits widespread use

• Three-plane motion via flexible keel or joints
• Better accommodation to uneven terrain
• K2 (limited community) ambulators

• Carbon fibre leaf-spring keel stores energy during loading phase; returns it as push-off during late stance
• Mimics the biological Achilles-calf mechanism
• Key designs: Seattle Foot, Carbon Copy II/III, Flex Foot, Ceterus Foot (with integrated shock absorber)
• K3-K4 ambulators; enables variable cadence and most normal activities
• Selection based on patient height, weight, activity level, and funding

• Real-time sensor-driven adjustment of stiffness and dorsiflexion
• K4 (high-impact) patients; expensive

Alignment

Bench Alignment

• Socket in 5-10° flexion to facilitate patellar tendon loading
• Ground reaction force passes approximately through the knee joint centre in mid-stance
• Foot aligned parallel to the contralateral foot

Dynamic Alignment

• Lateral trunk shift (varus/valgus socket)
• Knee instability (excessive socket flexion or foot plantar-flexion)
• Whip at toe-off (rotational malalignment at pylon-foot interface)
• Step length asymmetry (limb length error)

The PTB Cast: Non-Prosthetic Application

1. Tibial diaphyseal fractures (non-operative management): after initial long-leg cast immobilisation, a PTB cast allows knee mobilisation and early weight bearing while protecting the fracture. This exploits Sarmiento's principle that axial loading stimulates fracture healing through periosteal callus formation.
2. Post-amputation stump shaping: rigid dressings incorporating PTB principles help shape and mature the residual limb.

Special Considerations

Paediatric Amputees

Vascular and Diabetic Amputees

• Constitute the majority of lower limb amputees
• Skin fragility requires meticulous socket fit without focal pressure concentration
• Silicone liners preferred over hard direct-contact sockets
• Below-knee amputation (vs. above-knee) preserves the knee joint, reducing energy expenditure during walking by ~40% and dramatically improving rehabilitation potential
• The PTB prosthesis is specifically associated with high rates of successful community ambulation in this group

Complications

Rehabilitation Pathway

1. Pre-operative: Counselling, goal setting; prosthetist input on optimal stump length
2. Immediate post-operative (0-2 weeks): Rigid dressing or pneumatic post-amputation mobility (PPAM) aid; knee maintained in extension to prevent flexion contracture; drain for 24-48 hours
3. Stump maturation (weeks 2 - months 6-18): Figure-of-eight bandaging; volume stabilisation; physiotherapy for strengthening and proprioception
4. Temporary prosthesis: Fitted once wound healed and volume stabilising; allows gait training and builds socket tolerance
5. Definitive PTB prosthesis: Fabricated when volume stable; replaced as volume changes
6. Gait training: Parallel bars → walking aids → independent ambulation
7. Advanced rehabilitation: Sport-specific training; K4 prosthetic components where indicated

Energy Expenditure

• Normal walking: baseline
• Transtibial amputee with PTB prosthesis: approximately 25% increased energy expenditure
• Transfemoral amputee: approximately 65% increased energy expenditure
• Bilateral transtibial: approximately 40% increased energy expenditure

Conclusion

• Campbell's Operative Orthopaedics, 15th Edition, 2026 (Chapter 31: Amputations in Children)
• Miller's Review of Orthopaedics, 9th Edition (Chapter 10: Prosthetics and Orthotics)
• Rockwood and Green's Fractures in Adults, 10th Edition, 2025 (Chapter 21: Technical Considerations of Limb Amputation; Chapter 4: PTB Cast)
• Pye's Surgical Handicraft, 22nd Edition (Chapter 25: Amputations)
• Pfenninger and Fowler's Procedures for Primary Care, 3rd Edition (CPT 29435: PTB Cast Application)

Nitrogen Metabolism and Its Relationship to Purine and Pyrimidine Synthesis

Introduction

Part I: Nitrogen Metabolism - Overview and Core Pathways

1. Sources of Nitrogen

Figure 36.1 from Basic Medical Biochemistry (6th Ed): Amino acid nitrogen is used for urea synthesis; one nitrogen of urea comes from NH4+, the other from aspartate.

2. Transamination

• Alanine + α-ketoglutarate → Pyruvate + Glutamate (ALT/GPT)
• Aspartate + α-ketoglutarate → Oxaloacetate + Glutamate (AST/GOT)

3. Oxidative Deamination: Glutamate Dehydrogenase

Glutamate + NAD⁺ (or NADP⁺) → α-Ketoglutarate + NH₄⁺ + NADH

4. Ammonia Transport and Toxicity

• Glutamine: synthesised in most tissues by glutamine synthetase (glutamate + NH₃ + ATP → glutamine). The dominant ammonia-transport molecule from peripheral tissues to the liver and kidneys.
• Alanine: synthesised in muscle via transamination of pyruvate. Carries nitrogen to the liver while simultaneously delivering gluconeogenic substrate (glucose-alanine cycle).

5. The Urea Cycle

• NH₄⁺ (from glutamate dehydrogenase reaction) - incorporated first as carbamoyl phosphate
• Aspartate (the second nitrogen donor, entering the cycle directly)

Steps of the Urea Cycle:

• Occurs in mitochondrial matrix
• Requires N-acetylglutamate (NAG) as obligatory allosteric activator
• Rate-limiting step; irreversible
• CPS I uses nitrogen from free NH₄⁺
• (Note: CPS II, the cytosolic isoform, uses nitrogen from glutamine and produces carbamoyl phosphate for pyrimidine synthesis - a critical distinction)

• OTC is the most commonly deficient urea cycle enzyme (X-linked)
• Citrulline exits mitochondria in exchange for ornithine

• Aspartate donates the second nitrogen atom here
• Driven by ATP hydrolysis to AMP + pyrophosphate

• Fumarate enters the TCA cycle (via cytoplasmic fumarase → malate → oxaloacetate → transaminated back to aspartate, regenerating the nitrogen donor)

• Ornithine re-enters mitochondria to complete the cycle
• Urea is excreted by the kidneys

Part II: Nucleotide Structure and Nitrogen Content

Figure 22.3 from Lippincott Biochemistry (8th Ed): Comparison of purine (9-membered bicyclic ring, 4 nitrogens) and pyrimidine (6-membered ring, 2 nitrogens) base structures.

• Purines: Adenine (A) and Guanine (G) - bicyclic ring system with 4 nitrogen atoms
• Pyrimidines: Cytosine (C), Uracil (U), Thymine (T) - monocyclic ring with 2 nitrogen atoms

Part III: De Novo Purine Synthesis

Nitrogen Donors for Purine Ring Atoms

PRPP: The Activated Scaffold

Ribose-5-phosphate + ATP → PRPP + AMP

• Activated by inorganic phosphate
• Inhibited by purine nucleotides (end-product inhibition)
• X-linked gene

The Committed Step: GPAT Reaction

PRPP + Glutamine → Phosphoribosylamine + Glutamate + PPᵢ (catalysed by glutamine phosphoribosylpyrophosphate amidotransferase, GPAT)

• Inhibited by AMP and GMP (end-product feedback inhibition)
• Activated by PRPP (high PRPP signals need for more purines)

IMP to AMP and GMP (Cross-Regulation)

• IMP → AMP requires GTP (fumarate is released; aspartate donates N6)
• IMP → GMP requires ATP (glutamine amide donates N2)

Energy Cost

Part IV: De Novo Pyrimidine Synthesis

Nitrogen Donors for the Pyrimidine Ring

• N1: from Aspartate (the entire aspartate molecule enters the ring)
• N3: from Glutamine (amide nitrogen, via CPS II)
• C2: from CO₂ (via CPS II reaction)
• C4, C5, C6: from Aspartate

Key Enzymatic Steps

• CPS II (cytosolic) uses glutamine nitrogen - distinct from CPS I (mitochondrial) which uses free NH₄⁺ for the urea cycle
• CPS II activity is the regulated step; inhibited by UTP (end product), activated by PRPP

• In mammals, the first three enzymes (CPS II, ATCase, dihydroorotase) are present as a single trifunctional enzyme: CAD
• Dihydroorotate dehydrogenase is located on the inner mitochondrial membrane

• OPRT and ODC are also present as a bifunctional enzyme: UMP synthase

Pyrimidines for DNA: dTMP Synthesis

• dUMP → dTMP via thymidylate synthase (TS), which uses N⁵,N¹⁰-methylene-THF as the methyl donor
• This reaction oxidises THF to dihydrofolate (DHF); DHF reductase (DHFR) regenerates THF
• 5-Fluorouracil (5-FU) inhibits TS (suicide inhibitor); Methotrexate inhibits DHFR - both are anticancer drugs exploiting this pathway

Part V: The Critical Intersection - Carbamoyl Phosphate at the Crossroads

Figure from Basic Medical Biochemistry (6th Ed): When OTC is defective, excess carbamoyl phosphate diverts through the pyrimidine pathway, generating orotate excreted in urine.

Part VI: PRPP - The Central Hub Linking Both Pathways

• Required for de novo purine synthesis (GPAT reaction)
• Required for pyrimidine attachment to ribose (orotate + PRPP → OMP)
• Required for purine and pyrimidine salvage reactions
• Regulated by purine nucleotide concentrations (product inhibition)

Part VII: Purine Salvage Pathway

Figure 22.10 from Lippincott Biochemistry (8th Ed): Purine salvage via HGPRT (hypoxanthine → IMP; guanine → GMP) and APRT (adenine → AMP). Both reactions use PRPP and are irreversible due to pyrophosphate hydrolysis.

1. HGPRT (hypoxanthine-guanine phosphoribosyltransferase, X-linked): salvages hypoxanthine → IMP and guanine → GMP
2. APRT (adenine phosphoribosyltransferase): salvages adenine → AMP

Lesch-Nyhan Syndrome

• Inability to salvage hypoxanthine and guanine → both degraded to uric acid
• Elevated PRPP (not consumed by salvage) → increased de novo purine synthesis → more uric acid
• Decreased IMP/GMP → less feedback inhibition of GPAT → further increased de novo synthesis

Part VIII: Purine Degradation and Gout

• AMP → Inosine → Hypoxanthine → Xanthine → Uric acid (xanthine oxidase, XO)
• GMP → Guanosine → Guanine → Xanthine → Uric acid (XO)

Gout - Biochemical Basis

• Overproduction: HGPRT deficiency (Lesch-Nyhan), PRPP synthetase overactivity, high purine diet (meat, shellfish), excessive cell turnover (haematological malignancies, chemotherapy)
• Underexcretion (90% of primary gout): reduced renal urate excretion; alcohol and certain drugs (thiazides, low-dose aspirin) impair urate secretion

• Acute attack: anti-inflammatory agents (colchicine, NSAIDs, corticosteroids). Colchicine prevents microtubule polymerisation, reducing neutrophil migration into the joint.
• Long-term (urate-lowering):
  • Allopurinol: structural analogue of hypoxanthine; inhibits XO → accumulation of hypoxanthine and xanthine (more soluble than uric acid). Allopurinol is oxidised by XO to oxypurinol, a long-lived XO inhibitor (suicide substrate mechanism). In patients with normal HGPRT, hypoxanthine is salvaged back → reduces PRPP levels → reduces de novo purine synthesis (double benefit).
  • Febuxostat: non-purine XO inhibitor
  • Uricosuric agents (probenecid, sulfinpyrazone): increase renal uric acid excretion (for underexcretors)

Part IX: Deoxyribonucleotides - Reduction of Ribonucleotides

Ribonucleoside diphosphate (ADP, GDP, CDP, UDP) → Deoxyribonucleoside diphosphate (dADP, dGDP, dCDP, dUDP)

Part X: Clinical Connections and Pharmacological Exploitation

Summary of Nitrogen Flow

Dietary protein
      ↓ (digestion → amino acids)
Transamination (aminotransferases + PLP)
      ↓
Glutamate
      ↓ (GDH)                    ↓ (glutamine synthetase)
    NH₄⁺                       Glutamine
      ↓                              ↓
  CPS I (mitochondria)          CPS II (cytosol)
      ↓                              ↓
Carbamoyl phosphate          Carbamoyl phosphate
(urea cycle)                  (pyrimidine synthesis)
      ↓                              ↓
    UREA                          UMP → CTP → dCDP, TMP
   (excreted)

Aspartate → nitrogen into urea cycle AND into purine ring (N1) AND into pyrimidine ring (N1, C4-C6)
Glutamine → nitrogen into purine ring (N3, N9) AND into pyrimidine ring (N3, CTP synthesis)
Glycine → entire molecule into purine ring (C4, C5, N7)

Conclusion

• Lippincott Illustrated Reviews: Biochemistry, 8th Edition (Chapter 22: Purine and Pyrimidine Nucleotide Synthesis and Degradation)
• Basic Medical Biochemistry: A Clinical Approach, 6th Edition (Chapter 36: Fate of Amino Acid Nitrogen: Urea Cycle; Chapter 39: Purine and Pyrimidine Metabolism)