Tripolar Prosthesis

Definition and Concept

Components and Architecture

1. Femoral stem - standard cementless or cemented stem (identical to conventional THA)
2. Bipolar head assembly (inner articulation) - a metal femoral head, typically titanium nitride (TiN)-coated (ceramic-coated in modern designs), sitting inside a polyethylene inner shell. This constitutes the inner bearing (small head-in-liner motion, ~28 mm head)
3. Large-diameter acetabular component (outer articulation) - a hemispherical metal shell with a highly cross-linked polyethylene liner of large internal diameter (41-54 mm, median ~47 mm). The bipolar head unit articulates within this. This constitutes the outer bearing

Biomechanical Principles

Two Articulations, One Joint

• Inner articulation: femoral head rotates within the bipolar polyethylene shell
• Outer articulation: bipolar unit rotates within the large acetabular liner

Jump Distance

Head:Neck Ratio

Design Variants

Constrained tripolar designs have been largely abandoned due to mechanical failure and high revision rates.

Comparison: Tripolar vs. Dual Mobility vs. Conventional THA

Indications

1. Revision THA for recurrent dislocation - the primary indication
2. High-demand young active patients (adventure sports, physically demanding occupations)
3. Patients requiring unrestricted postoperative ROM (no activity restrictions)
4. Neuromuscular disorders with abductor weakness (Parkinson's, cerebral palsy)
5. Conversion of failed bipolar hemiarthroplasty with acetabular wear
6. Femoral failure after polyethylene hip resurfacing
7. Any patient where surgeon would otherwise use a constrained liner

Surgical Technique Highlights

• Can be performed via any standard approach (anterior, anterolateral, posterior)
• A conventional THA can be upgraded to tripolar simply by:
  • Adding a bipolar component over the existing femoral head
  • Replacing the polyethylene acetabular liner with a larger-diameter one
  • The shell and stem remain unchanged
• Minimally invasive/robotic-assisted techniques are compatible
• Direct anterior approach (DAA) is commonly used in modern practice

Outcomes and Evidence

• Mean UCLA activity score: 7.9 (high activity)
• Combined flexion/extension arc: 145° (vs. 119° for conventional THA)
• No osteolysis on radiographs
• No intraprosthetic dislocation
• Bipolar separation force: 2,180 N (versus ~340 N generated by high-demand activity - a safety margin of 6.4x)
• Implant survivorship supporting lifetime use even in highest-demand patients

• Patient-reported outcomes, sexual, and functional scores were higher than conventional THA
• Full unrestricted activity was permitted postoperatively
• No postoperative dislocation restrictions required

Advantages

1. Lowest dislocation rate of any unconstrained THA construct
2. Near-normal ROM without activity restrictions
3. Lowest wear - below the osteolytic threshold (~24 mm³/yr)
4. No intraprosthetic dislocation risk (unlike dual mobility cups)
5. No metal-on-metal wear reaction
6. Limited acetabular stress shielding
7. Can convert existing THA to tripolar without stem/shell revision
8. Compatible with all standard surgical approaches

Disadvantages and Complications

1. Not widely used - limited long-term multi-centre data
2. More complex construct than conventional THA
3. Requires larger acetabular shell to accommodate the large-diameter bipolar unit
4. Constrained designs (now obsolete) had high mechanical failure rates
5. Potentially higher polyethylene debris volume (though wear rate is actually low per retrieval data)
6. Increased operative complexity in index procedure

Relevant Image

Key Exam Points (Gold Medal Tips)

• The term "tripolar" refers to three poles of articulation: inner head, inner cup surface, outer cup surface
• The fundamental difference from a dual mobility cup: in DMC the small femoral head articulates inside a large mobile liner which in turn sits in the metal shell; in tripolar, a full bipolar prosthesis (its own separate articulating unit) is used as the inner bearing inside the outer cup
• Jump distance formula: proportional to femoral head diameter - tripolar achieves highest jump distance of all unconstrained designs
• Wear is paradoxically lower in tripolar than in conventional large-head THA because motion is shared across two surfaces, reducing excursion at each interface
• Constrained tripolar = mechanical failure; unconstrained tripolar = preferred design
• Primary indication in exams: recurrent post-THA dislocation and revision surgery

• Pritchett JW. Wear and performance of a tripolar total hip replacement. Ann Jt. 2022. PMID 38529131
• Pritchett JW. Adventure sports and sexual freedom hip replacement: the tripolar hip. EFORT Open Rev. 2017

Implant Design: Tripolar vs. Conventional THR

Conventional THR - Basic Architecture

Femoral Stem → Femoral Head → Polyethylene Liner → Metal Acetabular Shell
                    ↑
           Single articulation here

• Femoral head (28-36 mm) articulates directly against a fixed polyethylene liner
• The liner is locked into the acetabular shell and does not move
• Motion occurs at one point only - the head-liner interface

Tripolar THR - Architecture

Femoral Stem → Femoral Head → Bipolar Inner Shell → Large Poly Liner → Metal Acetabular Shell
                    ↑                    ↑
           Inner articulation    Outer articulation

• The femoral head (28 mm) rotates inside a polyethylene bipolar inner shell
• That entire bipolar unit (now ~47 mm effective diameter) rotates inside a large-diameter fixed polyethylene liner
• Motion is shared across two surfaces

Component-by-Component Comparison

The Critical Design Difference - Visually

[Stem]---[Head 28-36mm]<<<articulates against>>>[Fixed Liner]---[Shell]

• 1 moving interface
• Head is a solid ball against a fixed cup

[Stem]---[Head 28mm]<<<inner>>>[ Bipolar Shell ]<<<outer>>>[Large Liner]---[Shell]
         ↑ Inner articulation ↑              ↑ Outer articulation ↑

• 2 moving interfaces
• The bipolar shell floats between the head and the outer cup
• The head never directly contacts the outer liner

Design Consequences - Why It Matters

• Conventional 32 mm head: jump distance ~11 mm
• Conventional 36 mm head: jump distance ~12 mm
• Tripolar (~47 mm effective): jump distance ~16 mm
• Jump distance = the displacement needed to dislocate; larger = more stable

• In conventional THR, the neck can impinge against the liner rim at ~120° ROM
• In tripolar, the neck impinges against the inner bipolar rim first; the outer large cup is recessed and almost never reached
• Net effect: impingement-free arc is greatly extended

• Conventional: all wear concentrated at one surface
• Tripolar: wear distributed across two surfaces, each operating through smaller excursion angles
• Result: lower total volumetric wear (~24 mm³/yr vs ~38 mm³/yr for conventional 40 mm head)

• Conventional THR: combined flex/extension ~119°
• Tripolar: combined flex/extension ~145°
• Near-normal physiological ROM without risk of dislocation

What is Shared (No Design Difference)

• The femoral stem is identical to any standard cementless/cemented THA stem
• The acetabular metal shell outer geometry is the same (hemispherical, porous-coated for ingrowth)
• Fixation philosophy (cementless ingrowth, cemented) is unchanged
• Surgical approaches are the same - any standard approach works

Conversion Point (Exam Favourite)

1. Keep the existing femoral stem
2. Keep the existing acetabular metal shell
3. Swap the fixed liner for a larger-diameter HXLPE liner
4. Add a bipolar head assembly over the existing femoral head

Functional Braces in Orthopaedics

Definition

Biological and Biomechanical Rationale

1. Hydraulic Pressure Principle

2. Motion-Stimulated Healing

3. Muscle Pump Effect

Types of Functional Braces

A. Fracture Functional Braces

B. Ligamentous / Joint Functional Braces

Specific Applications

1. Humeral Shaft Fractures - The Gold Standard Use

• Initial 7-10 days: temporary immobilization (coaptation splint / U-slab / hanging cast) to allow oedema to subside
• Transition to functional brace once acute swelling settles
• Brace: two plastic shells (anterior-posterior or medial-lateral) secured with Velcro; applies compressive pressure to muscle belly
• Patient wears collar-and-cuff sling; elbow motion encouraged daily to prevent stiffness
• Pendulum shoulder exercises commenced immediately
• Patient taught to tighten Velcro straps as swelling subsides

• Up to 20° anterior/posterior angulation
• Up to 15° varus angulation
• Rotation is difficult to control - a key limitation

• Nonunion rate: 2.6% (1.5% closed, 5.8% open fractures)
• Average healing time: 9.5 weeks (closed), 14 weeks (open)
• 88.6% lost <10° shoulder motion; 92% lost <10° elbow motion
• Overall healing: ~93.5% across published series (Rockwood & Green, 10th ed, 2025)
• Average time to union across series: 10.7 weeks

2. Tibial Shaft Fractures

• Initial long-leg cast or well-moulded splint for 2-4 weeks (until swelling settles and callus begins)
• Transition to short-leg PTB (patellar tendon-bearing) cast or fabricated functional brace
• Advantage over PTB cast: allows both ankle and knee motion simultaneously
• Early weight-bearing is mandatory - it drives fracture healing through functional loading

• Open fractures (Grade II/III) - soft tissue damage prevents adequate circumferential compression and monitoring
• Initial shortening >12 mm (not correctable by bracing)
• Tibial fracture with intact fibula (increased risk of late varus angulation from fibular tether effect)
• Ipsilateral injuries preventing weight-bearing
• Fractures with initial displacement >30-50%

• Nonunion: 1.1%
• 94% healed with shortening ≤12 mm
• 90% healed with angulation ≤6°
• Average shortening at final follow-up: 4.3 mm (not significantly worsened by bracing/weight-bearing)

3. Ulnar Shaft Fractures (Nightstick Fracture)

• Functional bracing is highly effective for isolated ulnar shaft fractures
• Sarmiento reported 96.5% good and excellent results in the largest published series
• Significant advantage: preserves forearm rotation and prevents disuse atrophy

4. Knee Ligament Injuries / Post-operative Rehabilitation

• Functional braces are used after ACL reconstruction, collateral ligament repair, and meniscal surgery
• They provide proprioceptive feedback, protect the healing graft from valgus/varus/rotational stress, and allow progressive ROM
• Surgical textbooks (Schwartz's Principles of Surgery, 11th ed) recommend a functional brace for sports return after collateral ligament healing

5. Vertebral Compression Fractures / Spinal Fractures

• TLSO braces are a conservative option for stable thoracolumbar fractures
• Allow upright activity while offloading the anterior column

Indications Summary

Contraindications

1. Fractures with unacceptable initial alignment not correctible by closed means
2. Open fractures Grade II and above
3. Polytrauma where patient requires crutches (tibia) - weight-bearing impossible
4. Ipsilateral upper extremity fractures requiring crutch use (humerus)
5. Initial fracture shortening beyond acceptable limits
6. Intact fibula with tibial fracture (tether effect - varus risk)
7. Segmental fractures with significant displacement (relative)
8. Non-compliant patients

Advantages Over Plaster Cast

Complications

1. Varus angulation - particularly in transverse humeral fractures; caused by gravity on the unsupported elbow/arm
2. Rotational malunion of shoulder - loss of external rotation in up to 38-45% (under-recognized)
3. Nonunion - especially transverse fractures of the humerus (up to 9% in some series) and open tibial fractures
4. Skin problems - pressure sores, skin breakdown at plastic edges
5. Malunion - 32% in tibial closed treatment (systematic review data)
6. Brace loosening - as oedema resolves; requires patient compliance in tightening

Key Exam Points

• Sarmiento described functional humeral bracing in 1977 and tibial bracing in the 1960s
• Mechanism = hydraulic pressure of soft tissue sleeve + motion-stimulated callus
• The brace does NOT rigidly immobilize - it controls alignment while permitting functional activity
• For humerus: acceptable deformity = 20° anterior/posterior, 15° varus; valgus and rotation are less well controlled
• For tibia: intact fibula is a relative contraindication (varus tendency)
• Nonunion rates: humerus 2.6% (Sarmiento), tibia 1.1% (Sarmiento) - but other series show higher rates
• The major competitor for tibial fractures is IM nailing - RCTs and meta-analyses favour nail fixation over functional bracing for most tibial shaft fractures
• For isolated ulnar shaft fractures: 96.5% good/excellent - functional bracing is particularly effective here

• Rockwood and Green's Fractures in Adults, 10th ed (2025) - Chapters on Humeral Shaft and Tibial Shaft Fractures
• Campbell's Operative Orthopaedics, 15th ed (2026)
• Sarmiento A et al. - Humeral shaft functional bracing, JSES 2002 (PMID 11988725)
• Fox HM et al. Humeral shaft cost-effectiveness analysis, J Shoulder Elbow Surg 2022 (PMID 35398163)