Unicompartmental Knee Arthroplasty (UKA) features significantly lower overall complication rates compared to Total Knee Replacement (TKR) due to its bone-preserving, minimally invasive approach. However, UKA carries a unique clinical consideration absent in TKR: natural progression of osteoarthritis in the preserved compartments over time. This represents underlying disease evolution rather than technical failure and necessitates conversion to TKR in approximately 5%–10% of patients over a 10-year period.
Early Postoperative Complications (< 2%)
Early surgical complications following UKA are rare (aggregate rate < 2%) due to limited soft-tissue disruption and minimal blood loss.
- Deep Vein Thrombosis (DVT) & PE (0.5%–1%): Substantially lower than TKR; effectively prevented via chemical thromboprophylaxis and immediate postoperative weight-bearing.
- Superficial Wound Infection (< 1%): Most common in poorly controlled diabetic patients; typically resolves with targeted oral antibiotics.
- Periprosthetic Joint Infection (PJI) (< 1%): Rare due to minimal foreign material burden, though severe cases require standard staged revision protocols.
- Arthrofibrosis / Joint Stiffness (1%–3%): Far less common than in TKR; managed with structured rehabilitation or gentle manipulation under anesthesia (MUA).
- Periprosthetic Fracture: Extremely rare because minimal native bone stock is resected.
Late Complications and Implant Survivorship
- Disease Progression in Preserved Compartments: The most frequent late indication for intervention. Gradual cartilage wear in the untreated compartment occurs in 5%–10% of patients over 10–15 years, treatable via conversion to TKR.
- Component Malalignment: The primary technical complication, typically associated with non-navigated procedures. Suboptimal alignment accelerates poly wear or causes early aseptic loosening.
- Bearing Dislocation: Primarily seen in mobile-bearing (Oxford-type) designs following sudden rotational trauma or soft-tissue laxity, requiring bearing exchange and ligament rebalancing.
- Aseptic Loosening: Gradual debonding at the bone-cement-implant interface after decades of mechanical loading.
Prevention: How to Mitigate Surgical Risks
- Strict Patient Selection: Preserving UKA candidacy strictly for isolated unicompartmental OA with an intact ACL, correctable alignment, and no inflammatory arthropathy.
- Advanced Surgical Precision: Utilizing robotic assistance or computer navigation to ensure accurate coronal/sagittal component alignment and balanced gap kinematics.
- Comorbidity Optimization: Strict perioperative glycemic control (HbA1c optimization) and weight management to reduce joint contact pressures.
- Structured Physical Therapy: 12 to 20 sessions over 4 to 8 weeks to restore quadriceps activation, optimize joint biomechanics, and shield preserved compartments from asymmetric load transfer.
Red-Flag Symptoms Requiring Prompt Clinical Evaluation
- Fever (> 38.5°C), progressive warmth, or purulent wound discharge: Urgent evaluation for potential PJI.
- Calf pain, asymmetric swelling, or erythema: Immediate venous duplex ultrasound to rule out DVT.
- Persistent joint laxity or sensation of mechanical ‘giving way’: Assessment for component malalignment or insert displacement.
- Knee flexion restricted below 90° at 8 weeks post-op: Evaluation for arthrofibrosis.
- New localized pain in the unoperated compartment years later: Radiographic assessment for OA progression.
Frequently Asked Questions: Complications of Partial Knee Replacement
What is the overall complication rate for Partial Knee Replacement (UKA)?
Partial Knee Replacement is a highly successful, tissue-sparing procedure with an overall complication rate below 3% to 5%. Because the healthy compartments, bone stock, and cruciate ligaments (ACL/PCL) remain untouched, patients experience significantly lower blood loss, lower infection rates, and fewer systemic medical complications compared to Total Knee Replacement.
What is bearing dislocation and how does it happen?
Bearing dislocation or subluxation occurs primarily in mobile-bearing prostheses (e.g., Oxford UKA), where the unconstrained polyethylene insert dislodges from the polished tibial tray. Common mechanisms include:
- Sudden hyperflexion combined with rotational torque.
- Medial or lateral collateral ligament laxity.
- Improper gap balancing or component malposition during surgery.
Treatment involves prompt closed reduction under sedation or open surgical exploration with insert exchange.
How common is the progression of arthritis to the other knee compartments?
Progression of osteoarthritis into the un-operated compartments occurs in approximately 5% to 10% of patients over 15 to 20 years. If generalized degenerative arthritis develops, the partial knee can be straightforwardly revised to a primary Total Knee Replacement (TKR) because native bone and ligament structures were preserved during the initial surgery.
What causes aseptic loosening in a partial knee prosthesis?
Aseptic loosening is the non-infectious failure of fixation between the metal component and host bone. Major contributing factors include:
- Incomplete cement interdigitation or bone interface micro-motion.
- Excessive varus/valgus alignment stress and high mechanical impact.
- Underlying poor bone mineral density or severe osteopenia.
What is the risk of developing Deep Vein Thrombosis (DVT) after UKA?
The risk of symptomatic DVT following UKA is low (approx. 0.5%–1.0%). Because patients begin full weight-bearing on the day of surgery, venous stasis is significantly reduced. Prophylactic anticoagulation regimens, sequential compression stockings, and active calf-pump exercises further safeguard against thromboembolic events.
What are the warning signs of a prosthetic joint infection (PJI)?
Surgical site infections are uncommon (<0.5%) but require immediate medical attention if any of the following symptoms develop:
- Systemic fever (>38.3°C or 101°F) accompanied by chills.
- Progressive erythema, warmth, or localized induration around the incision.
- Persistent serosanguinous or purulent drainage from the surgical wound.
- Sudden escalation in knee pain with an acute decline in joint mobility.
Can unexplained persistent pain occur after Partial Knee Replacement?
A small subset of patients may report persistent joint-line discomfort. Common etiologies include unrecognized pre-existing chondromalacia in adjacent compartments, subtle soft-tissue impingement, pes anserine tendinopathy, or component overhanging irritating the collateral ligaments. Comprehensive clinical and radiologic evaluation allows targeted conservative or interventional management.
Is periprosthetic tibial fracture a risk during or after surgery?
Periprosthetic tibial plateau fractures are rare (<1%) and typically occur either intraoperatively from aggressive vertical sagittal cuts into the tibia or post-operatively secondary to high-impact falls before complete bone remodeling. Strict adherence to non-impact activities during the initial weeks minimizes this risk.
How do obesity and smoking elevate the risk of UKA complications?
Excessive body weight increases localized stress concentrations across the unicompartmental bearing, accelerating polyethylene wear and elevating the risk of component loosening. Tobacco use delivers nicotine and carbon monoxide, which severely restrict microvascular blood flow, significantly increasing the incidence of wound dehiscence and surgical site infections.
How can patients proactively minimize the risk of complications after UKA?
Patients can optimize their clinical outcomes and minimize complication risks through the following measures:
- Selecting an experienced, fellowship-trained orthopedic knee specialist.
- Strict adherence to prescribed thromboprophylaxis and early mobilization protocols.
- Participating in structured post-operative physical therapy for quadriceps strengthening.
- Maintaining a healthy BMI and refraining from high-impact sports.
- Attending all scheduled long-term clinical and radiographic follow-up appointments.
References
- AAOS OrthoInfo — Partial Knee Replacement: Complications and Conversion
- Cleveland Clinic — Partial Knee Replacement: Risks and Long-term Outcomes
- HSS — UKA to TKA Conversion: Outcomes and Success Rates
- Cleveland Clinic — Robotic Knee Replacement: Accuracy and Malalignment Prevention
- Medscape — UKA: Failure Modes and Revision Strategies