Modes of Lubrication - Bearing Surfaces in THR

Overview

1. Boundary Lubrication
2. Fluid Film Lubrication (with sub-types: Hydrodynamic, Elastohydrodynamic, Squeeze Film)

1. Boundary Lubrication

• A single monolayer of lubricant molecules is adsorbed onto each bearing surface
• The lubricant molecules physically attach to the surface and carry the load directly
• In natural joints, this is achieved by a macromolecular monolayer (lubricin / surface-active phospholipids) attached to the articular surface
• These layers directly contact each other, carrying loads and reducing friction
• Film thickness ~equals the roughness of the bearing surface (~0.3 mm)
• Operates predominantly at low velocities and high loads (when fluid film cannot be maintained)

Key feature: surfaces are essentially in contact; the lubricant prevents direct metal-on-metal or cartilage-on-cartilage adhesion.

2. Fluid Film Lubrication

• Viscosity of the lubricant
• Shape of the gap between the two bearing surfaces
• Relative velocity of the surfaces

Sub-types of Fluid Film Lubrication

a) Hydrodynamic Lubrication

• Motion at sufficiently high velocity tilts the bearing and forms a wedge shape of entrained lubricant
• Viscous properties of the fluid create pressure within the film to support the load
• Surfaces are fully separated - no solid-solid contact
• Works well in rigid bearing surfaces (e.g., metal-on-metal at high speeds)

b) Elastohydrodynamic (EHD) Lubrication

• The primary lubrication mechanism for articular cartilage during dynamic function
• Occurs when bearing materials are not rigid and stiff (as with cartilage)
• As surfaces move and entrain synovial fluid, the fluid pressure causes surface deformation (AC flattening)
• This deformation:
  • Increases the contact area
  • Increases load-carrying capacity
  • Reduces escape of lubricant from between surfaces
  • Generates a longer-lasting film
  • Produces lower stress concentration within joint surfaces
• Both hydrodynamic (sliding) and squeeze film (impact) actions can drive EHD lubrication

c) Squeeze Film Lubrication

• Occurs when two surfaces approach each other rapidly (e.g., at heel strike)
• Fluid is squeezed out from between the surfaces but the viscous resistance of the fluid generates a transient pressure that carries the load
• Very important at the start of motion and during high-impact loading before hydrodynamic film is established
• In deformable (EHD) bearings, surface deformation spreads the load over a larger area, prolonging the squeeze film effect

3. Mixed Mode Lubrication

• Fluid film pressure supports loads in areas of non-contact (troughs between surface asperities)
• Boundary lubrication operates in areas of contact (at surface asperities)

Comparison: Hydrodynamic vs. Squeeze Film (Rigid vs. Deformable)

Hydrodynamic Lubrication - Principle

Coefficient of Friction - Comparative Data

Role of Synovial Fluid

• Synovial fluid is thixotropic: becomes fluid when agitated, settles when at rest
• This property allows it to serve boundary lubrication needs at low velocities and fluid film needs at high velocities
• Joint effusion disrupts this velocity-dependent property, reducing lubrication quality and increasing surface damage
• Key molecular lubricants: hyaluronan, lubricin, and phospholipids - each plays a different role at different stages of motion, and none alone can explain the extremely low friction of natural joints

Summary Table

Bearing Surfaces in THR - Structured Answer (6 Marks)

A. THE FOUR BEARING COUPLES - Wear Rates

1. Metal-on-Polyethylene (MoP) - Conventional UHMWPE

• Backside wear - additional PE debris from PE insert micromotion against metal shell
• PE particles (submicron size) stimulate osteolysis when count exceeds 10 billion particles per gram of tissue
• Osteolytic lesions typically appear after 10 years with conventional PE

2. Metal/Ceramic-on-Highly Cross-Linked Polyethylene (HCLPE) - Most Popular Today

• Rates of osteolysis have fallen dramatically with widespread HCLPE adoption
• HCLPE allows larger-diameter femoral heads without excessive linear wear
• Wear rate largely independent of head diameter
• Disadvantage: Reduction in mechanical properties (fracture toughness, tensile strength) - catastrophic failure is extremely rare
• Free radical problem: Residual free radicals after irradiation cause oxidative degradation; addressed by:
  • Remelting (>135°C) - eliminates free radicals but reduces crystallinity
  • Annealing (below melting point) - preserves crystallinity but less effective
  • Vitamin E doping - scavenges free radicals without remelting stage
• Currently Ceramic-on-HCLPE is favored over Metal-on-HCLPE due to trunnion corrosion concerns

3. Metal-on-Metal (MoM)

• Run-in wear: Higher wear in first 1 million cycles (~1 year of high activity) - polishes out high points, carbide asperities, and areas out-of-round
• After run-in: lower steady-state wear rate
• Now largely abandoned due to ALTR (see Section C)

4. Ceramic-on-Ceramic (CoC)

• 1st generation alumina - head fracture rate up to 13.4% (due to neck impingement, adverse head-neck ratio, poor manufacturing)
• Modern AMC (delta ceramic): fracture rate ~1 in 100,000 (0.001%)
  • Compare to pure alumina: 1 in 5000 (0.02%)
• Ceramic head change: Any new ceramic head placed on a used femoral neck must use an internal titanium adapter sleeve - high points on roughened neck cause burst fracture
• Stripe wear - area of roughness from repetitive subclinical subluxation (head rotates on cup edge)
• After ceramic fracture: microscopic shards remain, severely abrasive - must replace with another CoC (use HCLPE if unavailable)

B. PSEUDOTUMOR AND ALTR in Metal-on-Metal (MoM)

Definition

1. Metal-on-metal articulating bearing surfaces
2. Modular neck femoral components
3. Stems with modular femoral heads (trunnion/taper corrosion)

Pathogenesis

• Nanometer-sized MoM particles dissolve to generate cobalt (Co²⁺) and chromium (Cr³⁺) ions
• Normal wear: serum Co and Cr levels mildly elevated (1-3 µg/L)
• ALVAL (Aseptic Lymphocyte-Dominated Vasculitis-Associated Lesion): Delayed hypersensitivity reaction to metal ions; CD4+ T-lymphocyte mediated
• Local tissue necrosis + cystic masses = pseudotumor
• More common in women (smaller head diameter, higher inclination, higher wear)

Metal Ion Levels and ALTR Risk (Campbell's Table 4.10)

Diagnosis

• Serum Co and Cr levels - first-line screening
• ESR, CRP - may be elevated in ALTR alone (mimics infection)
• Ultrasound or MARS MRI (Metal Artifact Reduction Sequence) - imaging gold standard for pseudotumor characterization

Contraindications to MoM

• Women of childbearing age (metal ions cross placenta)
• Renal failure (metal ions no longer eliminated)
• Metal hypersensitivity

Outcomes

• Results of ALTR surgery are generally poor - high rates of infection, instability, reoperation
• Refer to centers with required implant availability and surgical expertise

C. DUAL MOBILITY - THE BOUSQUET CONCEPT

Historical Origin

• Designed by Prof. Gilles Bousquet in France in 1974 (first implanted 1976)
• Concept: use two articulating interfaces to increase effective head-to-neck ratio and range of motion without increasing head size proportionally

Design Principle (The "Double Mobility" Concept)

Small metal head (22-28 mm)
         ↕  Inner articulation (small head in large PE ball)
Large PE outer ball ("mobile bearing" or "retentive cup")
         ↕  Outer articulation (large PE ball in metal shell)
Metal acetabular shell

• A small-diameter femoral head articulates with a large outer polyethylene liner (forming a large-diameter bipolar construct)
• The outer PE ball then articulates with the metal acetabular shell
• Two articulating surfaces = dual mobility

Biomechanical Advantages

• Provides greater impingement-free range of motion
• Larger head-to-neck ratio reduces dislocation risk dramatically

Clinical Applications

• Primary THA in high-risk dislocation patients: Poor abductors, neuromuscular disease, cognitive impairment
• Revision THA - historically the main indication
• Conversion of failed hemiarthroplasty - convert MoM monoblock to dual mobility on femoral side
• Low dislocation rates reported in both primary and revision settings

Complications

• Intraprosthetic dislocation (IPD): Dislocation of the inner head from the outer PE ball - requires open reduction - unique to dual mobility
• Outer bearing PE wear - long-term concern (outer articulation is metal-on-PE)
• Acetabular component: monoblock (no screws) or modular (ensure metal liner is properly seated)

D. TRUNNION CORROSION ("TRUNNIONOSIS")

Definition

Mechanism

• Fretting = micromotion at taper junction under cyclic loading
• Corrosion = electrochemical degradation of metal at the fretting site
• Combined process releases Co and Cr ions and metallic debris
• Can cause ALTR identical to MoM bearing failure

Contributing Factors (not fully elucidated)

• Large femoral head diameter (greater moment at taper)
• CoCr head on titanium stem (dissimilar metal galvanic corrosion)
• Increased patient activity / body weight
• Longer neck length (increased lever arm)
• Modular femoral neck junctions (additional taper = additional corrosion site)

Clinical Significance

• Concerns about trunnion corrosion have reduced use of CoCr femoral heads in favor of ceramic heads worldwide
• Metal debris from trunnion corrosion may be responsible for osteolysis appearing within first 2-3 years (vs. PE wear osteolysis which appears after 10 years)
• Management: revision to ceramic head with titanium adapter sleeve

E. CLINICAL APPLICATIONS OF TRIBOLOGY IN THR

Rotator Cuff Tears: Evaluation, Management & Arthroscopic Repair

KNRUHS Paper 4 Theory - 15 Marks Structured Answer

INTRODUCTION (1 mark)

I. CLASSIFICATION (2 marks)

A. By Depth

• Articular surface (undersurface) - most common; seen in throwing athletes and internal impingement
• Bursal surface - associated with extrinsic impingement
• Interstitial (intratendinous) - concealed within substance

B. By Size - Cofield Classification (Full-Thickness)

C. Fatty Infiltration - Goutallier Classification (MRI/CT)

Key point: Grade ≥3 fatty infiltration predicts poor repair outcomes

D. By Onset

• Acute traumatic - sudden injury, good tissue quality, repair within 3 weeks (outcomes significantly worse if delayed >4 months)
• Chronic degenerative - insidious, poor tissue quality, fatty infiltration

II. CLINICAL EVALUATION (3 marks)

A. History

• Age: >40 years; peak incidence 5th-6th decade
• Dominant complaint: Night pain (pathognomonic for rotator cuff pathology), painful arc (60-120°), weakness overhead
• Onset: Most patients cannot recall a specific traumatic incident
• Pain referred to area of deltoid insertion; pain distal to elbow suggests cervical radiculopathy

B. Physical Examination - Special Tests

C. Investigations

III. MANAGEMENT (2 marks)

A. Non-Operative Treatment

1. Activity modification - avoid overhead activities
2. NSAIDs - anti-inflammatory (note: preoperative NSAIDs predict inferior long-term outcome after repair)
3. Physiotherapy - strengthening periscapular muscles, rotator cuff rehabilitation, posterior capsule stretching
4. Subacromial corticosteroid injection - for symptomatic relief (max 3 injections per year)
5. PRP injection - emerging evidence; may improve non-operative outcomes

B. Operative Indications

• Acute traumatic full-thickness tears (especially in young, active patients)
• Failed conservative treatment >3-6 months
• Young patient (<63 years) with documented full-thickness tear
• Tear involving the rotator cable (anterior cable involvement)
• Progressive weakness or functional deterioration
• Professional/overhead athletes

IV. ARTHROSCOPIC REPAIR - TECHNIQUES (4 marks)

Principles of Arthroscopic Rotator Cuff Repair

• Provides superior visualization of glenohumeral joint and subacromial space
• Allows treatment of concomitant pathology (biceps, labrum, SLAP, ACJ)
• Lower morbidity, faster recovery versus open repair
• Equivalent clinical outcomes to open repair for all tear sizes

Patient Positioning

• Beach chair position - easier orientation, reduced neurovascular risk
• Lateral decubitus - better visualization for posterior tears, better traction

Portal Placement

• Posterior portal (primary viewing) - 2 cm inferior and 1 cm medial to posterolateral acromion
• Anterior portal - medial to bicipital groove
• Lateral portal - 3 cm distal to lateral acromion (working portal for supraspinatus repair)
• Accessory portals as required

Steps of Arthroscopic Repair

• Assess glenohumeral joint, biceps, labrum, articular-surface cuff pathology
• Identify tear: size, retraction, tissue quality

• Thorough bursectomy to visualize entire footprint
• Assess bursal surface tear extent

• Type III (hooked) acromion: routine acromioplasty
• RCT 2022 (Woodmass et al.) - long-term outcomes: no difference between repair with vs without acromioplasty in full-thickness tears
• Current trend: selective, not routine

• Release of coracohumeral ligament
• Posterior interval slide (for posterior supraspinatus and infraspinatus)
• Anterior interval slide (between supraspinatus and subscapularis)
• Release of capsule at glenoid insertion (glenoid margin incision)
• Critical: Do not dissect below teres minor (axillary nerve at risk in quadrangular space; suprascapular nerve at spinoglenoid notch)

• Decorticate greater tuberosity footprint with a shaver
• Create bleeding cancellous bone to enhance biological healing

Single-Row Repair

• Anchors placed in a single row along lateral aspect of greater tuberosity footprint
• Simple or mattress sutures through cuff to anchors
• Restores ~74% of footprint width
• Advantage: Simpler, faster, equivalent clinical outcomes to double-row in many studies

Double-Row Repair

• Medial row anchors: Placed at articular margin; mattress sutures through medial tendon edge
• Lateral row anchors: Placed at lateral footprint; simple sutures
• Restores 100% of native footprint area
• Better biomechanical strength and footprint contact
• Suture-bridge (transosseous equivalent - TOE) technique: Medial anchor sutures passed through cuff and tied, then suture limbs are passed over cuff and fixed to lateral knotless anchors - creates a "bridging" compression effect

• Biomechanically, double-row is superior (greater contact pressure, footprint area)
• Clinically: Multiple RCTs show equivalent functional outcomes for small-medium tears
• Double-row may be superior for large and massive tears (>3 cm) with regard to healing rates
• Retear rates: Double-row and suture-bridge have lower structural failure rates for large tears

Biceps Tendon Management

• Long head of biceps (LHB) pathology is common with RCT
• Options: tenotomy (quick recovery, older/low-demand) vs tenodesis (better cosmesis, younger/active)
• Biceps tenodesis increasingly preferred in patients <55 years

V. MASSIVE AND IRREPARABLE TEARS - OPTIONS (1.5 marks)

Definitions

• Massive tear: ≥5 cm, typically involves ≥2 tendons
• Irreparable tear: Cannot be brought to the footprint without excessive tension; Goutallier grade ≥3 fatty infiltration; severe muscle atrophy

Surgical Options for Irreparable Tears

• Latissimus dorsi transfer - for irreparable posterosuperior tears (infraspinatus/teres minor); contraindicated if subscapularis torn (loses humeral head centering)
• Pectoralis major transfer - for irreparable subscapularis tears
• Lower trapezius transfer - recent advance; better biomechanical alignment for external rotation restoration

• Introduced by Mihata et al. (2012)
• Fascia lata autograft or dermal allograft bridged from superior glenoid to greater tuberosity
• Acts as a static superior restraint, preventing superior migration
• Restores glenohumeral kinematics without requiring rotator cuff tissue
• Recent advance: Biceps tendon rerouting (LHB left attached proximally and distally, rerouted to GT anchor) as humeral head depressor - avoids graft donor site morbidity

• Biodegradable subacromial balloon injected arthroscopically
• Mechanically depresses humeral head, reduces superior migration
• Provides temporary pain relief (maintained up to 12-24 months until resorption)
• Emerging evidence for pseudoparalytic shoulder

• Definitive solution for massive irreparable tears with cuff tear arthropathy (Hamada/Neer classification)
• Deltoid substitutes for deficient rotator cuff
• Currently recommended for patients ≥55 years with low to moderate demands
• Newer literature supports use in younger active patients

VI. RECENT ADVANCES (2 marks)

1. Biological Augmentation

2. Arthroscopic Technology Advances

• 3D visualization systems - improved spatial orientation
• Robotic-assisted arthroscopy - in development
• Fluorescence imaging - assess tissue vascularity at repair site
• Intraoperative ultrasound - confirm repair completeness

3. Suture Anchor Evolution

• From metal anchors (titanium) to bioabsorbable anchors (PEEK, PLLA, β-TCP)
• All-suture anchors - smaller footprint, less cortical disruption, lower revision complication
• Knotless anchors - simplify arthroscopic technique, reduce knot-related complications

4. Repair Construct Evolution

• Transosseous equivalent (TOE) technique - maximizes footprint compression
• Knotless suture bridge - reduced construct bulk
• Rip-stop technique - additional mattress suture prevents medial suture cut-through in poor tissue

5. Decision-Making Tools

• MRI-based footprint mapping - predict repairability preoperatively
• Tear pattern classification (Gerber, Collin) - guides tendon transfer selection for irreparable tears
• Dynamic ultrasound - assessment of cable integrity and tear pattern

VII. REHABILITATION PROTOCOLS (0.5 marks)

VIII. COMPLICATIONS (0.5 marks)

SUMMARY TABLE - Examiner's Key Points

Your Notes - Marking Assessment

What you have written (honest evaluation):

• No Enneking staging / MST staging - examiner WILL expect this
• No indications and contraindications properly structured
• No specific tumor examples (osteosarcoma, Ewing's, etc.)
• No comparison with amputation (functional outcomes data)
• Flap coverage / vascular reconstruction thin
• Rehabilitation details absent
• Outcomes data / MSTS score not mentioned

COMPLETE MODEL ANSWER - Limb Salvage Surgery (10 Marks)

INTRODUCTION / DEFINITION (0.5 marks)

INDICATIONS AND CONTRAINDICATIONS (1 mark)

• Primary malignant bone tumors (osteosarcoma - most common, Ewing's sarcoma, chondrosarcoma)
• High-grade soft tissue sarcomas
• Aggressive benign tumors (Giant Cell Tumor - recurrent/aggressive)
• Metastatic disease with good prognosis (renal, breast, thyroid)

• Neurovascular bundle encasement (major vessels/nerves) - not just proximity
• Pathological fracture through tumor (contamination of entire compartment)
• Infection at operative site
• Inability to achieve adequate surgical margins
• Tumor extent that precludes functional reconstruction
• Patient preference / poor compliance

• Skeletally immature patient (expandable prosthesis has changed this)
• Major vessel involvement (now addressed with vascular reconstruction)

PREOPERATIVE EVALUATION (1 mark)

Staging - Enneking Staging System (Surgical Staging System - SSS)

Imaging Protocol

• MRI (Gold Standard) - extent of soft tissue involvement, pseudocapsule, skip lesions, neurovascular proximity, intramedullary extent; defines resection margins
• CT scan - cortical destruction, matrix calcification, pulmonary metastases
• PET-CT / Bone scan - rule out systemic metastases, multifocal disease, skip lesions
• Angiography (selective) - neurovascular involvement, feeding vessel identification, pre-embolization for hypervascular tumors

Biopsy Principles

• Biopsy MUST be performed in the operating center that will do the definitive surgery
• Core needle biopsy preferred (Trucut) - less contamination than open biopsy
• Biopsy tract included in resection specimen (must lie in the surgical field)
• Poorly planned biopsy is the most preventable cause of unnecessary amputation

GOALS OF LSS (0.5 marks)

1. Primary goal: Oncological clearance - tumor resection takes priority over reconstruction
2. Wide en-bloc excision with negative surgical margins
3. Sacrifice NV bundles if required for oncological clearance
4. Reconstruct defect to restore function

• Functional (painless, stable, allows ADL)
• Cosmetically acceptable
• Durable long-term

MULTIMODAL TREATMENT (0.5 marks)

• Reduces tumor volume, treats micrometastases
• Allows histological assessment of chemotherapy response (Huvos grade - predictor of survival)
• Grade III/IV response (>90% necrosis) = good prognosis
• Converts borderline resectable to resectable

SURGICAL PRINCIPLES (1 mark)

En-bloc Resection

• Tumor capsule integrity must be maintained at all times
• Wide margin: normal tissue cuff surrounding tumor on all sides
• Soft tissue defect managed with plastic surgery (rotational/free flap)
• Defect size does NOT compromise tumor resection - oncological margins not sacrificed for reconstruction convenience

Margin Classification (Enneking)

RECONSTRUCTION METHODS (3 marks)

Factors Influencing Choice

• Defect size and location (diaphyseal vs. epiphyseal vs. periarticular)
• Joint involvement
• Patient age and growth potential (skeletal maturity)
• Available resources and surgeon expertise
• Soft tissue coverage available
• Prognosis (life expectancy)

1. Prosthetic (Endoprosthetic) Reconstruction - Most Common

2. Biological (Osseous) Reconstruction

3. Hybrid Construct (Allograft-Prosthetic Composite - APC)

• Combines mechanical prosthesis with biological tissue (allograft)
• Example: Allograft-prosthetic composite for proximal femur/humerus
• Prosthesis provides immediate stability; allograft allows soft tissue/tendon reattachment

4. Arthrodesis (Fusion)

• Surgical fusion after tumor resection
• Durable, permanent, no wear or loosening
• Suitable for manual laborers, young active patients where prosthetic longevity is concern
• Disadvantage: Loss of joint mobility (significant functional limitation for hip/knee)
• Best for: Knee, ankle, shoulder (better function than flail joint)

5. Rotationplasty (Van Nes Procedure)

• Resection of distal femur/knee with 180° rotation of the lower limb
• Ankle joint acts as a biological knee joint after rotation
• Patient fitted with an external below-knee prosthesis
• Excellent functional outcomes in children, athletes
• Best indication: Distal femur / proximal tibia tumors in young children where growth plates must be spared
• Psychologically challenging (appearance) but functionally superior to above-knee amputation

6. Soft Tissue Reconstruction

• Rotational flaps (gastrocnemius, vastus lateralis) - local muscle coverage
• Free flaps (latissimus dorsi, rectus abdominis) - for large defects; requires microvascular anastomosis
• Plastic surgery collaboration is mandatory for complex soft tissue defects

COMPLICATIONS (1 mark)

Early Complications

Late Complications

OUTCOMES AND COMPARISON WITH AMPUTATION (0.5 marks)

• Pain, function, emotional acceptance, hand positioning/gait, lifting strength, walking ability

• 5-year survival: Equivalent between LSS and amputation for osteosarcoma (80-90% for localized disease with chemotherapy)
• Functional outcomes: LSS significantly superior on MSTS score
• Local recurrence: LSS ~5%, amputation ~2% - slightly higher but no difference in overall survival
• Quality of life: LSS superior - preserved body image, no phantom pain, better psychosocial outcomes

Marks Allocation Guide for Examiner