Total Knee Replacement Instability - Assessment and Constraint Ladder Management

Indication. Symptomatic instability after a total knee replacement — recurrent giving way (most often on stairs), mid-flexion instability, or a knee that feels loose — that has a correctable mechanical cause. Before any revision, diagnose the specific mode of instability, because a constrained implant is never a substitute for correcting the underlying problem. The mandatory gate — rule out infection first. Every unstable TKR is infected until proven otherwise. Before planning an instability revision:

• Bloods: ESR (greater than 30 mm/hr suspicious) and CRP (greater than 10 mg/L suspicious).
• Joint aspiration: cell count (greater than 3000 WBC suspicious), differential (greater than 80 percent PMNs suspicious), and culture held for 14 days for slow-growing organisms.
• Alpha-defensin if the picture is equivocal (high sensitivity and specificity). If infection is confirmed, abandon the instability plan and follow a two-stage protocol (resection, antibiotic spacer, reimplantation after eradication). Why the diagnosis drives the operation. Vince defined the four mechanical modes of instability — varus-valgus, recurvatum, flexion (mid-flexion) and global. Isolated polyethylene insert exchange and isolated ligament reconstruction generally do NOT work for true mechanical instability; of all the causes, only collateral ligament failure typically mandates a constrained implant. Most other causes are corrected by re-establishing alignment, sizing and component rotation first. Constraint is the last lever, not the first. Preoperative assessment (imaging and examination under anaesthesia).
• Standing radiographs: AP and lateral (component position, fixation, bone loss, joint space); a long-leg hip-knee-ankle alignment film (mechanical axis, normally 0 degrees plus or minus 3 degrees); a flexion weight-bearing PA (true joint space, polyethylene wear); a Merchant view (patellar tracking).
• CT (if available): component rotation (femoral relative to the epicondylar axis, tibial relative to the tubercle and transmalleolar axis) and Aori bone-loss quantification.
• Examination under anaesthesia: varus and valgus stress at 0 degrees and 30 degrees flexion (collateral competence — greater than 5 mm opening is incompetent), Lachman and posterior drawer, and a recurvatum test (posterolateral corner). Consent specifically for the possibility of needing a more constrained (CCK or hinge) implant than planned, blood loss and transfusion, infection, stiffness, common peroneal nerve palsy (especially with valgus correction or lengthening), extensor mechanism disruption, and the higher re-operation and loosening rates of constrained implants. Setup. Supine on a standard table, thigh tourniquet (inflate to 100 mmHg above systolic, maximum 2 hours), a leg holder or foot post so the knee can be taken freely from full extension to 90 degrees flexion for gap assessment; abduct the contralateral leg if a long-leg alignment check is needed.

The goal: expose the knee through the medial parapatellar approach, remove the failing components or polyethylene as required, correct alignment and rotation, balance the flexion-extension gaps into a rectangle, reconstruct any bone loss, and implant the minimum-constraint device that the collateral ligaments will support — escalating only when a collateral is incompetent. The exposure is laid out as the first steps below.

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Rehabilitation | Phase | Weight bearing | Range of motion | Therapy | |-------|---------------|-----------------|---------| | 0 to 2 weeks | WBAT if cemented; PWB if cones or graft; TDWB if extensor repair or tubercle osteotomy | CPM 0 to 60 degrees day 1, increase 10 degrees daily toward 0 to 90 degrees by week 2 | Quad sets, straight leg raises, ankle pumps | | 2 to 6 weeks | Progress per fixation | Passive and active-assisted ROM; cautious with constrained implants | Gait training, isometric to isotonic strengthening | | 6 to 12 weeks | Wean the assistive device | Full ROM as stability allows | Proprioception, balance, advanced strengthening | | 3 to 6 months | Full | Maintenance | Return to function | Cautious, slower ROM is used for rotating-hinge and constrained implants because of the higher stress. Return to desk work is typically 6 weeks; heavy manual work 12 to 16 weeks. Anticoagulation and antibiotics. Extended DVT prophylaxis for 4 to 6 weeks (LMWH enoxaparin 40 mg daily, or a DOAC — rivaroxaban 10 mg daily or apixaban 2.5 mg twice daily), longer than the 2 weeks typical of a primary TKR because revision is high-risk. Antibiotics for 24 to 48 hours per institutional protocol (cefazolin or vancomycin). Surveillance. Wound check and X-ray at 6 weeks, then 3 months, 6 months and 1 year, and annually for life thereafter — monitoring for radiolucent lines (greater than 2 mm and progressive is concerning for loosening), osteolysis, subsidence and wear. Constrained implants warrant especially close annual surveillance. Complications

Epidemiology. Instability is among the leading indications for revision total knee replacement. Its modes were classified by Vince (2006): varus-valgus, recurvatum, flexion (mid-flexion) and global. The risk and the achievable outcome scale with the constraint needed — posterior-stabilized revisions do best, rotating-hinge salvage the worst — which is why the minimum-necessary-constraint principle is the organising idea of the whole operation. The modes of instability and their cause. Most instability is mechanical and correctable: component malposition (rotation, varus-valgus, flexion-extension, tibial slope — the commonest correctable cause), ligament insufficiency (the MCL above all), flexion-extension gap mismatch, polyethylene wear, or extensor-mechanism weakness. Vince's central teaching is that isolated insert exchange and isolated ligament reconstruction generally do not work for true mechanical instability; only collateral ligament failure typically mandates a constrained implant, and everything else is fixed by re-establishing alignment, sizing and rotation.

Constraint survival — why the ladder matters. The reason for stepping up only as far as needed is that higher constraint transfers more load to the bone-implant interface and loosens faster. Posterior-stabilized knees survive 90 to 95 percent at 10 years with only 2 to 5 percent loosening at 5 years. Modern second-generation CCK series report 90 to 94 percent mid-term survival (Mancino 2020: 93.6 percent at 9 years, infection rather than loosening the main failure mode) — better than the 75 to 85 percent quoted for older constrained designs. Rotating-hinge survival is lower again: pooled meta-analysis (Abdulkarim 2019) gives about 82 percent non-tumour and 69 percent tumour survival at 6 to 10 years, with infection — not loosening — the single leading failure mode at roughly 30 percent of failures. The hinge also limits ROM to about 0 to 90 degrees versus 0 to 115 degrees for a PS knee. This gradient is exactly why a rotating hinge is a salvage procedure for the unreconstructable knee, not an ideal primary solution.