Total Knee Arthroplasty (TKA) — Implant Designs

Total knee arthroplasty (TKA) is one of the most performed elective orthopaedic operations worldwide, with over 1 million procedures annually in the United States alone. It reliably reduces pain and improves function in end-stage knee arthritis. Understanding implant design principles, alignment concepts, fixation, and complications is fundamental knowledge for every orthopaedic surgeon.

• Primary indication: end-stage knee osteoarthritis (90%); also rheumatoid arthritis, post-traumatic arthritis, avascular necrosis, inflammatory arthropathy
• Patient selection: significant functional limitation, failure of conservative management (physiotherapy, analgesia, weight loss, walking aids, intra-articular injection), radiological evidence of joint space loss
• Oxford Knee Score (OKS): 12-question patient-reported outcome measure; 0–48 (48 = best); standard outcome tool in the UK; used pre- and post-operatively; score ≤26 typically considered threshold supporting surgical intervention
• Contraindications: active infection (absolute), neuropathic joint (relative), extensor mechanism deficiency, severe peripheral vascular disease, young highly active patient (relative — UKA or osteotomy preferred)
• 10-year survivorship: approximately 95%; 20-year survivorship approximately 85–90% for well-fixed, well-aligned implants

• More constrained = more force transferred to bone-implant interface = higher risk of aseptic loosening; use minimum constraint necessary to achieve stability — the guiding principle of implant selection
• PS vs CR: no definitive superiority of one over the other in primary TKA with intact PCL — surgeon preference and training largely determines choice; PS provides more predictable flexion gap and range of motion

Achieving appropriate alignment is central to implant longevity and function. The traditional paradigm of neutral mechanical alignment is now challenged by kinematic alignment and personalised approaches.

• Mechanical axis: hip-knee-ankle (HKA) angle; target 0° ± 3° in mechanical alignment — deviations beyond ±3° (HKA outliers) associated with increased loosening, polyethylene wear, and early revision in multiple registry studies
• Kinematic alignment: supported by multiple RCTs showing non-inferior survivorship and superior patient satisfaction at 5–7 years; long-term survivorship data (15+ years) still accumulating; growing adoption worldwide
• Tibial slope: posterior tibial slope of approximately 3–7° is standard; excessive slope increases PCL tension and posterior tibial translation in CR TKA; insufficient slope reduces flexion; critical in ACL-deficient patients

• Whether to resurface the patella remains one of the most debated topics in TKA — practices vary significantly between countries and surgeons
• Arguments for resurfacing: eliminates native patellofemoral arthritis; avoids anterior knee pain from residual patellar disease; consistent meta-analysis evidence that resurfacing reduces rate of re-operation for anterior knee pain
• Arguments against: complications of resurfacing (patellar fracture, component loosening, avascular necrosis); not necessary in all patients; selective resurfacing of diseased patellae achieves equivalent outcomes
• UK practice: approximately 50% resurface routinely; NICE guidance does not recommend for or against as evidence is balanced; most RCTs show no significant difference in pain or function but higher re-operation rate without resurfacing for anterior knee pain
• Patellar clunk syndrome: a fibrous nodule forms on the posterior quadriceps tendon and catches on the intercondylar notch of PS implants during extension from deep flexion; produces painful clunk; treated arthroscopically by nodule excision
• Patellar tracking: assess throughout range of motion after implant placement — no-thumb test (lateral retinaculum released if patella subluxes without thumb pressure laterally); medialise tibial component or lateralise patella component if tracking poor

• Cemented TKA: gold standard for most patients; immediate fixation; excellent long-term survivorship; Knee Society Registry data consistently favours cemented fixation in patients over 65
• Cementless TKA: requires good bone stock for osseointegration; press-fit porous or HA-coated components; growing evidence of equivalence to cemented in younger patients; avoids cement disease; technically demanding
• Cross-linked polyethylene (XLPE): standard bearing in modern TKA — dramatically reduces wear rate compared to conventional PE; reduces osteolysis; highly oxidised XLPE (Vitamin E stabilised) further improves oxidation resistance; titanium tibial trays with porous coating preferred for cementless fixation
• Tibial component: all-polyethylene tibial component vs metal-backed — all-poly has lower manufacturing cost, equivalent results in older low-demand patients; metal-backed preferred for most TKA especially with modular PE exchange options

• Computer-assisted surgery (CAS/navigation): reduces outlier alignment (HKA >3° from neutral); consistent evidence of improved mechanical alignment accuracy; no definitive improvement in functional outcomes or survivorship in large registry studies; time-consuming; pins required
• Patient-specific instrumentation (PSI): MRI or CT pre-operative planning generates custom cutting blocks; reduces outlier alignment; no improvement in OR time in most studies; expensive; benefits disputed in meta-analyses
• Robotic-assisted TKA (MAKO, ROSA, Navio): most rapidly growing technology; pre-operative plan with intraoperative adjustment; superior alignment accuracy; reduces outlier rate; early evidence of improved functional outcomes and patient satisfaction at 1–2 years; long-term survivorship data still accumulating; significant cost and learning curve
• NJR and international registry data: no definitive survivorship benefit for navigated or robotic TKA vs conventional at 10 years; however, earlier functional improvement data and reduced outlier rates support adoption in high-volume centres

• Unexplained pain after TKA: the most common reason for dissatisfaction; exclude PJI (aspiration, serology, CRP/ESR), aseptic loosening (weight-bearing X-rays), malalignment (long leg alignment films), patellofemoral problems; psychological factors and sensitisation contribute significantly — assess before revision surgery
• Patient dissatisfaction rate: approximately 15–20% of TKA patients report suboptimal outcomes despite technically successful surgery — unmet expectations, psychological comorbidity, and pre-operative chronic pain sensitisation are major contributors

• Indications for UKA (Oxford criteria): anteromedial OA; intact ACL; correctable deformity; BMI <35; no inflammatory arthritis; flexion >90°; fixed flexion <15°
• UKA advantages: bone preservation; faster recovery; better proprioception and range of motion; lower early mortality than TKA; revision to TKA remains an option
• UKA survivorship: approximately 80–85% at 10 years (NJR) — lower than TKA; higher revision rate but revision is generally easier than TKA revision; Oxford Phase 3 UKA: 95% at 10 years in high-volume centres
• High tibial osteotomy (HTO): preferred in young (<55), active, varus OA with intact medial compartment; corrects mechanical axis to offload medial compartment; preserves native joint; buys time before arthroplasty

• Flexion gap vs extension gap balancing: the central technical challenge of TKA; symmetric rectangular gaps in flexion and extension are the goal; tight flexion gap = limited ROM and posterior tibial translation; tight extension gap = flexion contracture; asymmetric gaps = instability or malalignment; soft tissue releases guided by deformity pattern
• Coronal deformity correction: varus deformity — sequential medial release (deep MCL first, then superficial MCL, then posteromedial capsule); valgus deformity — lateral release (ITB, posterolateral capsule, popliteus; PCL release in severe valgus); over-release risks instability requiring increased constraint
• Posterior condylar offset: restoration of posterior condylar offset (PCO) is critical for post-operative flexion — insufficient PCO reduces flexion by reducing posterior rollback; augmented posterior condyles used in revision to restore PCO
• Revision TKA complexity: staged approach — identify failure mode (PJI, loosening, instability, malalignment, PE wear, fracture); manage infection with two-stage protocol; augment bone defects (AORI classification for bone loss — types I, II, III) with metal augments, cones, sleeves, or allograft; use more constrained implant to address ligament deficiency
• Manipulation under anaesthesia (MUA): perform within 3 months of TKA for flexion <90° — after 3 months, fibrosis matures and MUA less effective and higher fracture risk; ensure adequate analgesia and physiotherapy immediately after MUA to maintain gained flexion

• More constraint = more force to bone-implant interface = higher loosening risk; use minimum constraint necessary
• PS TKA: cam-post replaces PCL; used when PCL diseased, deficient, or in flexion contracture
• Mechanical alignment: HKA 0° ±3°; outlier alignment (>3°) = increased loosening and PE wear
• Kinematic alignment: restores native joint line; RCT evidence of non-inferior survivorship and improved satisfaction at 5–7 years
• XLPE: standard bearing; 80% wear reduction vs conventional PE
• Patellar clunk: fibrous nodule catches on PS intercondylar box during extension; arthroscopic excision
• MUA: within 3 months for flexion <90°; after 3 months — fibrosis mature, higher fracture risk, less effective
• Varus deformity correction: medial release (deep MCL first); valgus: lateral release (ITB first)
• Unexplained pain post-TKA: exclude PJI, loosening, malalignment, patellofemoral problems; 15–20% patient dissatisfaction even after technically successful surgery
• UKA: 10-year survivorship 80–85% NJR; lower than TKA but revision to TKA straightforward