Strength-Duration (SD) Curve in Orthopedics

What is the Strength-Duration Curve?

Figure: A (left) - Strength-duration curve showing the inverse relationship between stimulus intensity and duration. B (right) - Corresponding action potential with absolute and relative refractory periods. (Medical Physiology, Boron & Boulpaep, p. 276)

Physiological Basis

Law of Excitation (Imax Law / Weiss's Law)

• A brief, high-intensity stimulus is equivalent in effect to a long, low-intensity stimulus.
• Below a critical duration (the utilization time), no contraction is possible regardless of intensity (vertical asymptote).
• Below a critical intensity (the rheobase), no contraction occurs regardless of duration (horizontal asymptote).

Lapicque's Equation (1907)

I = b (1 + c/d)

• I = threshold current
• b = rheobase
• c = chronaxie
• d = stimulus duration

Key Parameters

1. Rheobase

• Pulse duration used to measure rheobase: 100-300 ms
• Greater rheobase = lower excitability = more denervation

2. Chronaxie

3. Utilization Time

Types of SD Curves - Diagnostic Patterns

Type 1: Normal Innervation ("Nerve Curve")

• Shape: Smooth, continuous rectangular hyperbola
• Curve begins to rise sharply at approximately 1 ms - the critical point where nerve can no longer be stimulated efficiently
• At longer durations (>1 ms): Intensity plateaus (rheobase reached)
• Rheobase: 2-18 mA / 5-35 V
• Chronaxie: < 1 ms
• A brisk, snap-like (vermicular) contraction is produced

Type 2: Complete Denervation ("Muscle Curve")

• Curve is shifted far to the right
• Steep rise continuing even below 100 ms - no plateau at short durations
• No response at short pulse durations (< 1 ms)
• Very high rheobase (may need 50-100+ mA)
• Chronaxie: > 10 ms (often 20-100 ms)
• Contraction is slow, worm-like (peristaltic), not brisk
• Requires galvanic (DC) current, not faradic (AC)

Type 3: Partial Denervation ("Kink Curve")

• A characteristic kink (inflection point) is visible in the curve
• Left side of the curve (short durations): represents intact innervated fibers - curves like a normal SD curve
• Right side of the curve (longer durations): represents denervated muscle fibers - shifted right with high intensity needed
• The kink appears where the two populations of fibers have different thresholds

Summary Comparison Table

Clinical Applications in Orthopedics

1. Diagnosis of Nerve Injury / Denervation

• Peripheral nerve injuries (trauma, stretch, laceration)
• Brachial plexus injuries
• Radiculopathies (disc prolapse, spinal stenosis, spondylosis)
• Compartment syndrome with nerve compromise
• Thoracic outlet syndrome

2. Monitoring Re-innervation / Recovery

• Kink appearing in a previously flat denervated curve = early re-innervation (nerve fibers are returning to the muscle)
• Kink shifting left over time = progressive re-innervation
• Return of normal curve pattern = complete re-innervation
• This is often detectable weeks before clinical strength recovery or EMG changes become apparent

3. Lower Motor Neuron vs. Upper Motor Neuron Lesions

• SD curve shows abnormality ONLY in lower motor neuron (LMN) lesions (anterior horn cell, nerve root, peripheral nerve)
• It is normal in upper motor neuron (UMN) lesions (stroke, spinal cord injury above level) because peripheral motor nerves remain intact
• This makes it a useful differentiating tool

4. Guiding Therapeutic Electrical Stimulation

• Use pulse durations around chronaxie for maximum efficiency and patient comfort
• Denervated muscles require much longer pulse widths (10-1000 ms galvanic) vs. innervated muscles (0.1-1 ms faradic)
• Prevents unnecessary fatigue and patient discomfort by avoiding ineffective short pulses

5. Prognosis and Medico-legal Assessment

• A right-shifted curve or high chronaxie = poor prognosis for spontaneous recovery
• Return of the kink = favorable sign of re-innervation
• Serial testing provides objective documentation of nerve recovery progress, useful in medico-legal contexts for compensation assessment

6. Radiculopathy Assessment

How the Test is Performed

1. Equipment: Galvanic stimulator capable of delivering rectangular pulses from 0.01 ms to 1000 ms
2. Electrodes: Active electrode (cathode, negative) placed over the motor point of the muscle; indifferent electrode (anode) placed proximally
3. Procedure:
   • Start with long-duration pulses (300-1000 ms) to find rheobase
   • Progressively reduce pulse duration while increasing intensity to find minimum threshold at each duration
   • Plot threshold intensity vs. duration on log-log or semi-log paper
4. Timing: Performed 3 weeks post-injury; repeated every 4-6 weeks to monitor progress

Factors Affecting the SD Curve

Advantages of the SD Curve

• Quick, inexpensive, non-invasive (relatively)
• Requires minimal training
• Can detect denervation before EMG changes become manifest in some cases
• Serial testing objectively tracks re-innervation
• Distinguishes complete from partial denervation
• Differentiates LMN from UMN lesions

Limitations

• Provides qualitative rather than quantitative grading of degree of denervation
• Cannot localize the site of the nerve lesion
• In large muscles, only a small area of fibers is sampled
• Patient cooperation and pain tolerance affect results
• Not reliable in < 3 weeks post-injury
• Largely superseded by EMG/nerve conduction studies for precise lesion characterization, though still used as a complementary tool and in resource-limited settings
• Intrarater reliability is moderate (r = 0.541) while interrater reliability is better (r = 0.945)

Comparison with Other Electrodiagnostic Tests

Summary

1. Diagnose peripheral nerve injury and denervation (from 3 weeks post-injury)
2. Monitor recovery and re-innervation serially over time
3. Differentiate complete from partial denervation (the "kink" pattern)
4. Guide therapeutic electrical stimulation parameters
5. Prognosticate nerve recovery in peripheral nerve and root injuries

• Campbell's Operative Orthopaedics, 15th ed. (2026), Electrical Stimulation section
• Medical Physiology (Boron & Boulpaep), p. 276, Figure 7-3
• Physiopedia: Strength-Duration Curve
• Akulwar-Tajane et al., "Exploring the Role of Strength-Duration Curve in Radiculopathy," EC Orthopaedics 14.9 (2023)

Biopsy Techniques in Orthopedics

Introduction and Why It Matters

• Major diagnostic errors in 13.5% of patients
• Complication rate of 15.9%
• Unnecessary amputations in 3%
• Problems 2-12x more frequent when biopsy was performed at non-specialist centers

"Errors in diagnosis, nonrepresentative or technically poor biopsy specimens, and wound complications have been found to occur more frequently when the biopsy was done by a surgeon at a referring institution rather than by an orthopaedic oncologist." - Campbell's Operative Orthopaedics, 15th ed. (2026)

Pre-Biopsy Planning (Mandatory Steps)

The 10 Golden Rules of Orthopedic Biopsy (Mankin's Principles)

1. Refer before you biopsy - if primary malignancy is suspected, send to a musculoskeletal oncologist first
2. The biopsy track is contaminated - treat both needle and open incision tracks as permanently seeded with tumor cells
3. The track must be excised en bloc with the tumor at definitive surgery
4. Incision placement is critical - must lie within the planned limb salvage incision
5. Longitudinal incisions only - transverse incisions on limbs are absolutely forbidden
6. Single compartment approach - never violate intermuscular planes
7. Avoid major neurovascular structures at all costs
8. Meticulous hemostasis - hematoma dissects tumor cells through tissue planes
9. Plugging of bone windows with methylmethacrylate cement to prevent fracture and hematoma
10. The biopsy surgeon = the definitive surgeon (or must communicate in intimate detail)

Classification of Biopsy Techniques

BIOPSY TECHNIQUES
│
├── PERCUTANEOUS (Minimally Invasive)
│   ├── Fine-Needle Aspiration Cytology (FNAC)
│   ├── Core Needle Biopsy (Tru-Cut / Jamshidi)
│   └── Image-Guided (CT / Ultrasound / Fluoroscopy)
│
└── OPEN (Surgical)
    ├── Incisional Biopsy (Gold standard tissue yield)
    └── Excisional Biopsy (Diagnosis + treatment together)

• Trephine Biopsy (bone marrow)

Type 1: Fine-Needle Aspiration Cytology (FNAC)

Principle

Indications

• Strongly suspected metastatic bone disease (95% accuracy)
• Suspected osteomyelitis / infection (for culture + sensitivity)
• Lymph node assessment
• Known primary tumor: rapid confirmation of recurrence
• Lesions very close to neurovascular bundles (thin needle = minimal contamination)

Technique

1. Clean skin with antiseptic
2. Local anesthetic to skin and subcutaneous tissue (do not over-inject - extends contamination zone)
3. 22-25G needle on a 20 mL syringe introduced directly into tumor
4. Strong suction applied while making 3-4 passes through tumor in different directions
5. Release suction before withdrawing needle (prevents aspirate going back into syringe)
6. Material expressed onto glass slides, smeared, and fixed immediately
7. Staining: Giemsa, Papanicolaou (Pap), or Diff-Quik

Accuracy

• Distinguishing malignant vs. benign: ~90%
• Specific tumor type identification: 60-70% (limited without architecture)
• Metastatic disease: ~95% (most accurate application)

Advantages

• Outpatient, no incision
• Essentially zero track-seeding risk (< 1 mm needle)
• Rapid results (same day)
• Very low complication rate
• Cheap

Disadvantages

• No tissue architecture (cannot grade primary tumors)
• Cannot perform immunohistochemistry, cytogenetics, or flow cytometry reliably
• Requires a specialist cytopathologist
• Not adequate for primary bone sarcomas as the sole biopsy

Type 2: Core Needle Biopsy (CNB)

Needles Used

Principle of Tru-Cut Biopsy: (1) Sheath and inner needle with specimen slot positioned at lesion margin. (2) Inner needle pushed into lesion - tumor enters the slot. (3) Outer sheath slides forward over the inner needle, cutting the tissue core. (4) Entire assembly withdrawn with specimen trapped.

• A spring-loaded gun mechanism reduces crushing artifact and patient discomfort
• Disposable guns available commercially
• Needle size: typically 14-18 gauge

Bone biopsy trephines: (a) Jamshidi single-bevel needle - most widely used, (b) Triple-bevel crown needle - better for hard sclerotic bone, (c) Ayka single-bevel needle. All consist of cannula + stylet; stylet is removed after cortex penetration to collect the core.

Technique - Soft Tissue (Tru-Cut Gun)

1. Plan needle trajectory to be within planned surgical incision corridor
2. Minimal local anesthetic (over-infiltration widens contamination zone)
3. Small stab skin incision with No.11 blade
4. Spring-loaded biopsy gun introduced until tip at tumor margin
5. Safety released, gun fired: inner needle advances into tumor, sheath cuts core
6. Entire assembly withdrawn; core removed from slot and placed in formalin
7. Repeat 2-3 times through the same skin puncture at slightly different angles
8. One core kept fresh (unfixed) for cytogenetics/molecular studies

Technique - Bone (Jamshidi)

1. Stab incision in line with planned surgical incision
2. Jamshidi needle (with inner trocar locked) directed to bone cortex
3. Rotatory drilling motion (clockwise + counterclockwise alternating) to penetrate outer cortex
4. Feel for loss of resistance when marrow cavity entered
5. Inner trocar removed; cannula advanced further into tumor
6. Rotate cannula to core out tissue
7. Withdraw while rotating; use stylet/extractor to push specimen out from tip end
8. Plug cortical hole with bone wax or PMMA cement
9. Take 2-3 cores

Image Guidance

Accuracy of CNB

• Overall diagnostic accuracy: 84-98% across multiple studies
• Malignant vs. benign determination: 97% (series of 252 patients)
• Specific diagnosis and grade: 81%
• CT-guided series (Ghelman): 85% histologic diagnoses
• CT-guided series (Kattapuram et al.): 92% accurate diagnoses
• Metastatic disease: 95% (most accurate)
• Benign primary tumors: ~82% (least accurate - may need open biopsy)

Advantages over Open Biopsy

• Outpatient procedure (no general anesthesia needed)
• Lower complication rate
• Preserves tissue architecture (unlike FNAC)
• Multiple cores from single skin puncture
• Smaller contamination zone
• Faster recovery

When CNB Should Be Escalated to Open Biopsy

• Non-diagnostic CNB (repeat CNB first; if still non-diagnostic - open biopsy)
• Cartilaginous lesions (grading chondrosarcoma requires architecture and cellularity assessment over large area)
• Primary lymphoma of bone (needs ample tissue for immunophenotyping)
• When ancillary studies require more tissue than cores provide

Type 3: Incisional (Open) Biopsy

Definition

Indications

• CNB non-diagnostic or inconclusive
• Large primary bone tumors needing grading
• Cartilaginous tumors (chondrosarcoma grading)
• Lymphoma of bone
• Complex tumors requiring cytogenetics, flow cytometry, molecular studies
• Surgeons performing definitive surgery at the same center

Technique - Step by Step

• Always longitudinal along the limb axis
• Centered over the most accessible/prominent part of the tumor
• Shortest path to tumor
• Must be placeable within the definitive surgical incision

• Through a single muscle belly (e.g., through vastus lateralis, not between vastus and rectus)
• Never through intermuscular planes (these connect to multiple compartments - wide contamination)
• Avoid neurovascular bundles

• Sample the leading edge (soft-tissue extension) - most viable, most representative
• Avoid the center of large tumors (necrotic, non-diagnostic)
• Avoid sampling just the pseudocapsule
• Take 2-3 pieces of representative tissue

• Create a round or oval defect - NOT rectangular, not irregular
• Round/oval windows distribute mechanical stress uniformly, preventing pathologic fracture
• Rectangular windows create stress risers at corners
• Plug with methylmethacrylate (PMMA) after sampling to prevent hematoma and reduce fracture risk

• Send tissue to pathologist intraoperatively
• Confirms: "Is this diagnostic tumor tissue?"
• If negative: take more tissue before closing

• Release tourniquet and achieve perfect hemostasis before closing (contaminated hematoma dissects tumor cells everywhere)
• Close tightly in layers
• No wide retention/tension sutures
• If drain needed: exit in line with the wound (NOT through a separate stab far from the incision)

What NOT to Do

Campbell's Fig. 26.8 - Errors to avoid: (A,B) Irregular oval bone defects causing pathological fracture. (C) Transverse incision - absolutely forbidden. (D) Needle track through patellar tendon. (E) Posterior needle track impossible to resect. (F) Multiple needle tracks contaminating quadriceps. (G) Drain placed separately from incision, creating a second contaminated corridor.

Type 4: Excisional Biopsy

Definition

Indications

• Small lesions (< 3 cm) in expendable bones: fibula head, rib, small bones of hand/foot, clavicle, iliac crest
• Lesions where imaging very strongly suggests benign (osteoid osteoma, fibrous cortical defect, small enchondroma)
• Small superficial soft-tissue masses radiologically consistent with lipoma

Risks and "Whoops" Lesion

• No pre-operative staging (compromised)
• A contaminated pseudocapsule (margins likely inadequate)
• Need for re-excision with much wider margins, possible radiation, or amputation

Type 5: Trephine Biopsy (Bone Marrow Biopsy)

Specific Application

Site: Posterior Superior Iliac Spine (PSIS)

Technique - Jamshidi at PSIS

1. Patient prone or lateral decubitus
2. PSIS identified and marked (most accessible, greatest marrow volume)
3. Local anesthetic - skin, subcutaneous, and periosteum (most painful; wait 2-3 minutes)
4. Small skin incision with No. 11 blade
5. Jamshidi needle with stylet locked - advanced to bone, then rotated through cortex
6. Once cortex penetrated (sudden loss of resistance): advance 1-2 cm into marrow
7. Rotate 360° CW then CCW to sever specimen from surrounding marrow
8. Withdraw slowly while rotating counterclockwise
9. Extractor rod inserted through the top to push specimen out distally (not proximally - avoids crushing)
10. Specimen should be 1.5-2.5 cm long for adequate assessment

Aspirate + Trephine Together

• Aspirate first (before trephine traumatizes marrow architecture): 0.5-2 mL into EDTA tubes
  • Aspirate for: morphology, flow cytometry, cytogenetics (karyotype/FISH), molecular studies
• Trephine after: for histology, IHC, gene rearrangement studies on fixed tissue

Specimen Handling - Universal Rules

• Always send with clinical and imaging summary for the pathologist
• Label clearly: patient ID, site, orientation if relevant
• Do not crush with forceps - use atraumatic handling

Complications

Summary Table

Key Mnemonics

• Longitudinal only
• One compartment (single muscle)
• Not near neurovascular structures
• En-bloc excision planned for entire track

• Round or Oval defect only (never rectangular)
• Avoid stress risers
• Defect plugged with PMMA immediately

• Campbell's Operative Orthopaedics, 15th ed. (2026), Chapter 26, pp. 1091-1092
• Mankin HJ et al. "The hazards of the biopsy, revisited." J Bone Joint Surg Am. 1996;78(5):656-663
• Agarwal M et al. "Biopsy for Musculoskeletal Tumors." J Clin Orthop. 2016
• Anatomically Based Guidelines for Core Needle Biopsy of Bone Tumors. Radiographics 2007;27(1)
• Biopsy of bone tumors: a literature review. PMC 11164262 (2024)