Unicompartmental Knee Replacement: Who is the Ideal Candidate?

Unicompartmental Knee Replacement: Who is the Ideal Candidate?

Introduction & Epidemiology

Unicompartmental knee replacement (UKR), also known as partial knee arthroplasty, involves resurfacing only the arthritic compartment of the knee while preserving the healthy cartilage and ligamentous structures in the remaining compartments. Historically introduced in the 1970s, UKR initially faced high revision rates, leading to a decline in popularity. However, advancements in implant design, surgical technique, and particularly, refined patient selection criteria have significantly improved outcomes, leading to a resurgence in its use.

UKR stands in contrast to total knee arthroplasty (TKA), which replaces all three compartments (medial, lateral, and patellofemoral). The primary objective of UKR is to restore knee kinematics as close to native physiology as possible, offering potential advantages over TKA, including smaller incision, less bone resection, preservation of cruciate ligaments (often both ACL and PCL), faster rehabilitation, and potentially higher patient satisfaction due to a more "normal" feeling knee. Its use is typically driven by isolated, end-stage osteoarthritis (OA) of the medial or, less commonly, the lateral compartment.

Epidemiologically, knee OA affects a significant portion of the adult population, with its prevalence increasing with age and obesity. While TKA remains the gold standard for global, multi-compartmental arthritis, approximately 20-30% of patients presenting for knee arthroplasty may have truly isolated unicompartmental disease, making them potential candidates for UKR. The utilization of UKR varies geographically, often reflecting surgeon training and regional preferences. Current trends indicate a modest but growing proportion of knee arthroplasties performed as UKRs, particularly in centers with specialized expertise.

Surgical Anatomy & Biomechanics

A thorough understanding of knee anatomy and biomechanics is paramount for successful UKR. The knee is a complex synovial joint comprising three functional compartments: medial tibiofemoral, lateral tibiofemoral, and patellofemoral.

Tibiofemoral Compartments

• Medial Compartment: The most commonly affected compartment in OA, often due to a physiological varus alignment and disproportionate load bearing. Articulations involve the medial femoral condyle and the medial tibial plateau. Stability is augmented by the medial collateral ligament (MCL) and posteromedial capsule.
• Lateral Compartment: Less frequently affected by isolated OA. Articulations involve the lateral femoral condyle and the lateral tibial plateau. Stability is maintained by the lateral collateral ligament (LCL) and the posterolateral corner structures (popliteus tendon, arcuate complex).

Cruciate Ligaments

• Anterior Cruciate Ligament (ACL): Essential for anterior-posterior stability and control of tibial rotation. Its integrity is critical for the long-term success of most UKR designs, particularly in maintaining normal tibiofemoral kinematics and preventing anterior tibial subluxation.
• Posterior Cruciate Ligament (PCL): Contributes to posterior stability and roll-back during flexion. While often preserved in UKR, its role is less directly implicated in UKR stability compared to the ACL, especially in fixed-bearing designs. Its integrity is generally beneficial for optimal kinematics.

Menisci

The menisci (medial and lateral) are fibrocartilaginous structures that enhance joint congruity, distribute load, absorb shock, and contribute to stability. In unicompartmental OA, the affected meniscus is typically degenerated, extruded, or severely damaged, necessitating its removal during UKR. The contralateral meniscus must be intact and functional.

Biomechanics of UKR

UKR aims to replicate native knee biomechanics by preserving healthy structures.
1. Kinematics: By retaining both cruciate ligaments, UKR allows for more physiological tibiofemoral roll-back and rotation, contributing to a more natural feel and range of motion compared to PCL-substituting TKAs.
2. Load Distribution: Preservation of the uninvolved compartments, including articular cartilage and menisci, allows for physiological load sharing across the remaining knee, theoretically reducing stress on the implanted components and improving overall joint longevity.
3. Joint Line Preservation: The minimal bone resection involved in UKR helps maintain the native joint line, which is crucial for balanced ligament tension and patellofemoral mechanics. Alterations in the joint line can lead to stiffness, instability, or patellofemoral dysfunction.
4. Ligamentous Balance: Achieving proper ligamentous balance is paramount. Over-resection of bone can lead to instability, while under-resection can result in impingement or stiffness. Correct tensioning of the MCL (for medial UKR) or LCL (for lateral UKR) within the physiological range is critical for stability in both flexion and extension.

Indications & Contraindications

The selection of an ideal candidate for UKR is arguably the most critical factor influencing long-term success. Strict adherence to established criteria is essential to optimize outcomes and minimize complications.

Key Principles for Ideal Candidacy:

1. Isolated Compartmental Disease: This is paramount. Radiographic and clinical evidence must confirm that symptoms arise solely from one tibiofemoral compartment.
2. Ligamentous Integrity: Stable knee with intact anterior and posterior cruciate ligaments, and competent collateral ligaments.
3. Correctable Deformity: Fixed flexion contracture should be minimal (<10-15 degrees). Varus (for medial UKR) or valgus (for lateral UKR) deformity should be passively correctable to neutral or slight overcorrection, implying no significant contracted collateral ligament.
4. Intact Patellofemoral Joint: No significant chondrosis, osteophytes, or symptomatic patellofemoral arthritis.
5. Patient Factors: Realistic expectations, compliance with rehabilitation, and a generally healthy physiological status.

Indications

Operative Indications (Criteria for UKR)

• Pain: Localized, reproducible pain in the affected tibiofemoral compartment, unresponsive to conservative management.
• Radiographic Evidence:
  • Kellgren-Lawrence Grade III-IV osteoarthritis primarily confined to one tibiofemoral compartment on weight-bearing radiographs.
  • Presence of joint space narrowing, osteophytes, subchondral sclerosis, and/or subchondral cysts limited to the affected compartment.
  • Long-leg alignment views demonstrating a mechanical axis that passes through the diseased compartment, but correctable to neutral or slight overcorrection on stress views.
• Ligamentous Stability:
  • Intact ACL and PCL, confirmed by clinical examination (Lachman, pivot shift, posterior drawer tests).
  • Stable MCL and LCL with no significant laxity in extension or 30 degrees of flexion. Varus/valgus stress views may be beneficial in equivocal cases.
• Patellofemoral Joint: Asymptomatic and radiographically healthy (no significant joint space narrowing, severe osteophytes, or chondromalacia on skyline views or MRI).
• Range of Motion: Good pre-operative range of motion (>100 degrees flexion, <10-15 degrees fixed flexion deformity).
• Age/Activity Level: While initially reserved for older, lower-demand patients, modern UKRs have expanded to younger, active patients given improved implant designs and long-term data. However, careful consideration of activity demands is still warranted.
• Body Mass Index (BMI): Historically, BMI >30 kg/m² was a relative contraindication. Contemporary literature suggests acceptable outcomes for patients with BMI up to 35-40 kg/m², but higher BMI patients may have increased complication rates and slightly inferior outcomes.
• Bone Stock: Adequate bone stock in both the femur and tibia to support implant fixation.

Non-Operative Indications (Factors suggesting UKR is NOT appropriate or less ideal)

Pre-Operative Planning & Patient Positioning

Meticulous pre-operative planning is essential to ensure precise component placement, achieve proper alignment, and anticipate potential surgical challenges.

Pre-Operative Planning

1. Clinical Assessment:
   • History: Detailed pain history (localization, exacerbating/alleviating factors), functional limitations, previous knee surgeries, medical comorbidities.
   • Physical Examination: Crucial for confirming isolated compartment disease and ligamentous integrity.
     • Gait analysis: Assess for varus/valgus thrust.
     • Range of motion: Document flexion and extension.
     • Palpation: Confirm pain localization.
     • Ligamentous testing: Stress tests for MCL/LCL at 0 and 30 degrees, Lachman, anterior/posterior drawer, pivot shift test for ACL/PCL integrity. Cruciate ligament integrity is non-negotiable for most UKR systems.
     • Patellofemoral assessment: Palpation, grind test, tracking.
2. Radiographic Series:
   • Weight-bearing Anteroposterior (AP) view: Assesses joint space narrowing, osteophytes, sclerosis in both tibiofemoral compartments. Flexion views (e.g., 30-45 degrees) can better visualize posterior condylar wear.
   • Lateral view: Assesses osteophytes, posterior condylar wear, and fixed flexion deformity.
   • Patellofemoral (Merchant/Skyline) view: Evaluates patellofemoral joint space, patellar tilt, and tracking. Essential to rule out symptomatic patellofemoral OA.
   • Long-leg standing AP (full-length mechanical axis) view: Critical for assessing overall limb alignment, identifying the true mechanical axis, and quantifying varus/valgus deformity. It also helps confirm that the contralateral compartment is healthy.
   • Varus/Valgus Stress views (optional but recommended for equivocal cases): Performed at 0 and 30 degrees of flexion to assess correctability of deformity and quantify ligamentous laxity. An uncorrected deformity implies fixed contracture and potentially suitability for TKA.
   • MRI (optional): Not routinely required but can be useful in ambiguous cases to confirm cartilage status, assess meniscus integrity in the contralateral compartment, or evaluate cruciate ligament status if clinical exam is inconclusive. It can also identify occult pathologies.
3. Templating:
   • Utilizing radiographs (calibrated if digital) or specialized software, template the femoral and tibial components to predict appropriate sizes and confirm bone resection levels.
   • Plan for restoration of the joint line and neutral or slight valgus (medial UKR) or varus (lateral UKR) alignment.
   • Consider different implant designs (fixed vs. mobile bearing, cemented vs. cementless) based on surgeon preference and bone quality.
4. Implant Selection:
   • Fixed-bearing: More forgiving in terms of surgical technique for ligamentous balance, often preferred in cases of subtle ligamentous laxity.
   • Mobile-bearing: Requires precise ligamentous balance to prevent bearing dislocation; theoretically offers lower polyethylene wear and more physiological motion. Requires an intact ACL and PCL.
   • Cemented vs. Cementless: Cemented implants are the gold standard. Cementless options are evolving, particularly with modern porous coatings, and may be considered in younger, active patients with good bone quality.

Patient Positioning

1. Supine Position: The patient is positioned supine on the operating table.
2. Tourniquet: A pneumatic tourniquet is typically applied high on the thigh to facilitate a bloodless field, which is crucial for cement technique and visualization. The tourniquet cuff should be adequately padded.
3. Leg Holder/Support: The operative leg is often placed in a specialized leg holder or supported by an assistant. This allows for controlled flexion, extension, and rotation during the procedure. Ensure the hip is slightly flexed and externally rotated to allow for full knee flexion.
4. Padding: All pressure points, especially the contralateral limb and ulnar nerves, must be meticulously padded to prevent neuropathies.
5. Preparation and Draping: The limb is prepped from the iliac crest to the toes, including the foot, using an antiseptic solution. Sterile draping isolates the operative field, allowing for free manipulation of the limb throughout the range of motion.

Detailed Surgical Approach / Technique

The goal of UKR is to achieve accurate implant alignment and restoration of normal limb mechanics while preserving the uninvolved compartments. The following outlines a general approach for medial UKR, which is the most common. Lateral UKR principles are similar but require specific anatomical considerations.

General Principles

• Minimally Invasive: While specific "MIS" UKR techniques are described, the principle is to minimize soft tissue dissection, not just skin incision length.
• Bone Preservation: Remove only the necessary bone to fit the implant and correct the deformity.
• Accurate Alignment: Restore the mechanical axis and achieve precise component rotation.
• Ligamentous Balance: Ensure appropriate tension in the MCL/LCL throughout the range of motion.
• Cruciate Ligament Preservation: Protect the ACL and PCL.

Medial Unicompartmental Knee Arthroplasty (UKA) Technique

1. Incision:
   • A straight longitudinal incision, typically 8-12 cm in length, centered over the medial aspect of the knee. It can be slightly paramedian or medial to the patella.
   • Incision length should be sufficient for adequate exposure without excessive retraction.
2. Approach (Medial Parapatellar/Mid-Vastus/Sub-Vastus):
   • Medial Parapatellar: A traditional approach, involves incising the medial capsule and potentially mobilizing the patella laterally. Care must be taken to minimize disruption of the extensor mechanism.
   • Mid-Vastus: Incision into the vastus medialis obliquus.
   • Sub-Vastus: Dissection deep to the vastus medialis obliquus. Often preferred for its muscle-sparing nature, potentially leading to faster recovery of quadriceps strength. Requires careful identification and protection of the saphenous nerve and its infrapatellar branch.
3. Exposure:
   • Retract the patella and extensor mechanism to provide visualization of the medial tibiofemoral compartment.
   • Excise the osteophytes from the medial femoral condyle and tibial plateau. This is crucial for accurate gap balancing and preventing impingement.
   • Perform a complete meniscectomy of the diseased medial meniscus.
4. Tibial Preparation:
   • Tibial Resection Guide Placement: Position a provisional guide for the tibial cut. The goal is to resect sufficient bone to remove the arthritic surface while minimizing overall bone loss, typically 4-6mm depending on implant design. The resection should allow for optimal thickness of the polyethylene insert and restoration of the joint line.
   • Varus/Valgus Alignment: The tibial cut is typically perpendicular to the mechanical axis of the tibia in the coronal plane, but some surgeons aim for 2-3 degrees of varus to achieve slight overcorrection for medial UKR.
   • Posterior Slope: A slight posterior slope (3-7 degrees) is typically incorporated, matching the native tibial slope or implant design, which aids in PCL function and femoral roll-back.
   • Resection: Use an oscillating saw to make the planned cut. Check with a feeler gauge for accuracy.
   • Punch/Keel Preparation: Prepare the cancellous bone for the tibial keel or pegs using appropriate instruments.
5. Femoral Preparation:
   • Sizing and Resection Guide Placement: Position the femoral sizing guide on the medial femoral condyle. Select the appropriate size to match the contour of the condyle without oversizing (leading to impingement) or undersizing (leading to poor coverage).
   • Resection: Perform the distal and posterior cuts of the medial femoral condyle using specific guides. The cuts should remove the diseased cartilage and subchondral bone, creating a clean surface for the implant. The goal is to match the resected bone to the thickness of the femoral component.
   • Anterior/Posterior Reference: Ensure the femoral component is neither too anterior nor too posterior, which can affect patellar tracking and flexion stability.
   • Drilling/Pegs: Drill holes for the femoral component pegs or prepare for a cemented surface.
6. Gap Balancing and Trial Reduction:
   • Crucial Step: Insert trial femoral and tibial components with a trial polyethylene insert of appropriate thickness.
   • Extension Gap: Assess stability and alignment in full extension. Ensure the MCL is adequately tensioned without being overtightened or lax.
   • Flexion Gap: Assess stability and alignment at 90 degrees of flexion. Check for appropriate PCL tension and no impingement.
   • Range of Motion: Test the full range of motion. The knee should move smoothly with no clunking or subluxation.
   • Balance: The primary goal is to achieve a rectangular flexion and extension gap. If the gap is too tight, consider minor additional bone resection or release of marginal osteophytes. If too loose, a thicker polyethylene insert is needed. Avoid excessive soft tissue release of the MCL, which can lead to instability.
   • Patellar Tracking: Ensure the patella tracks centrally without significant lateral tilt or subluxation.
7. Definitive Component Implantation:
   • Bone Preparation: Clean and dry the bone surfaces thoroughly.
   • Cementing: Apply bone cement (PMMA) to the prepared bone surfaces and the undersurface of the implants (for cemented designs). Insert the femoral component, then the tibial component with the appropriate polyethylene insert.
   • Pressurization: Apply firm, sustained pressure to ensure good cement interdigitation.
   • Excess Cement Removal: Meticulously remove all excess cement from around the components and within the joint to prevent impingement, wear, or intra-articular debris.
   • Final Assessment: Re-check range of motion, stability, and patellar tracking.
8. Closure:
   • Irrigate the joint thoroughly.
   • Close the capsulotomy and muscle layers (e.g., vastus medialis) with absorbable sutures.
   • Close subcutaneous tissue and skin.
   • A drain is often not necessary.

Lateral Unicompartmental Knee Arthroplasty (LUKA)

• Incisions and Approaches: Similar principles, but a lateral parapatellar, mid-vastus, or sub-vastus approach is used. Requires careful attention to protecting the peroneal nerve, which is more superficial laterally.
• Bone Resection: Similar cuts on the lateral femoral condyle and lateral tibial plateau.
• Balance: Focus on LCL tension and the posterolateral corner structures. Varus stress release of the IT band may be needed for fixed valgus deformities. LUKA is generally more technically demanding due to variable anatomy and lower surgical volume.

Robotic-Assisted UKR

Robotic-assisted systems (e.g., MAKO, NAVIO) are increasingly used in UKR. These systems utilize pre-operative CT scans to create 3D bone models, allowing for precise planning of bone resections and component positioning. Intra-operatively, the robot guides the surgeon's resections, enhancing accuracy and potentially improving alignment consistency. While evidence for superior long-term outcomes over conventional techniques is still accumulating, robotic assistance can reduce outliers in component positioning and alignment.

Complications & Management

While UKR offers potential advantages, it is not without risks. Complications can be intraoperative, early post-operative, or late. A thorough understanding of these complications and their management is crucial for all orthopedic surgeons.

General Management Principles:

• Prevention: Meticulous surgical technique, strict patient selection, appropriate DVT prophylaxis, and perioperative antibiotic administration.
• Early Recognition: Prompt identification of complications through clinical vigilance and appropriate investigations (e.g., plain radiographs, aspiration, blood tests).
• Tailored Approach: Management must be individualized based on the specific complication, patient comorbidities, and implant characteristics. Conversion to TKA is a common salvage pathway for failed UKR.

Post-Operative Rehabilitation Protocols

A well-structured and consistent post-operative rehabilitation protocol is crucial for maximizing functional outcomes, minimizing complications, and achieving patient satisfaction after UKR. The emphasis is on early mobilization, pain control, and restoration of strength and range of motion.

Immediate Post-Operative Phase (Day 0 - Week 2)

• Pain Management: Multimodal analgesia (NSAIDs, acetaminophen, nerve blocks, opioids as needed) to facilitate early mobility.
• Weight-Bearing: Immediate weight-bearing as tolerated (WBAT) with crutches or a walker is typically allowed for cemented UKRs. For cementless implants, some surgeons may prefer protected weight-bearing for 4-6 weeks, though early WBAT is increasingly common.
• Mobilization:
  • CPM (Continuous Passive Motion) machine: May be used to assist with gentle range of motion, though its routine use is debated and not universally indicated.
  • Early Active and Passive ROM: Gentle knee flexion and extension exercises are initiated immediately.
  • Ankle Pumps: To promote circulation and reduce DVT risk.
• Muscle Activation:
  • Quadriceps Sets: Isometric contractions of the quadriceps.
  • Gluteal Sets: Isometric contractions of the gluteal muscles.
• Edema Control: Elevation, ice, and compression.
• DVT Prophylaxis: Continued pharmacological (e.g., aspirin, LMWH) and mechanical (e.g., TED hose, SCDs) prophylaxis according to institutional guidelines.
• Wound Care: Monitor incision for signs of infection or hematoma.
• Discharge Criteria: Adequate pain control, ability to ambulate safely with assistive device, independence with transfers, and achievement of initial ROM goals (e.g., 0-90 degrees).

Early Rehabilitation Phase (Weeks 2 - 6)

• Physical Therapy Focus: Progressive increase in range of motion and strength.
• Range of Motion:
  • Active and passive knee flexion/extension exercises.
  • Patellar mobilizations to prevent stiffness.
  • Goal: Achieve near-full extension and at least 110-120 degrees of flexion.
• Strengthening:
  • Progressive resistive exercises for quadriceps (e.g., straight leg raises, mini-squats, wall slides), hamstrings (e.g., hamstring curls), and gluteal muscles.
  • Emphasis on controlled eccentric movements.
• Gait Training: Wean off assistive devices as strength and balance improve. Focus on normalized gait pattern.
• Proprioception/Balance: Single-leg standing, balance board exercises.
• Modalities: Continue ice and elevation as needed for swelling.

Intermediate Rehabilitation Phase (Weeks 6 - 12)

• Advanced Strengthening:
  • Increase resistance and repetitions for all lower extremity muscle groups.
  • Functional exercises: lunges, step-ups/downs, partial squats.
  • Core strengthening.
• Cardiovascular Endurance: Stationary cycling, swimming, elliptical trainer.
• Balance and Agility: More challenging balance exercises, lateral movements.
• Activity Progression: Gradual return to light recreational activities (e.g., walking, golf).
• Patient Education: Discuss long-term care, activity restrictions (avoidance of high-impact sports).

Advanced Rehabilitation Phase (Weeks 12 onwards)

• Functional Return: Return to higher-level activities and work.
• Sport-Specific Training: If appropriate, for patients returning to low-impact sports.
• Maintenance Program: Encourage continuation of home exercise program for long-term strength and flexibility.
• Long-term Follow-up: Regular clinical and radiographic follow-up to monitor implant integrity and joint status.

Summary of Key Literature / Guidelines

The body of literature on UKR has significantly expanded, providing robust evidence for its efficacy and delineating criteria for optimal patient selection.

UKR vs. TKA Outcomes

• Patient Satisfaction: Multiple studies and meta-analyses suggest that patients undergoing UKR often report higher satisfaction rates and a more "natural-feeling" knee compared to TKA. This is attributed to the preservation of native kinematics, bone stock, and cruciate ligaments.
• Functional Outcomes: UKR typically results in a faster recovery, earlier return to activities, and often better range of motion post-operatively than TKA. Functional scores (e.g., Oxford Knee Score, WOMAC) frequently show superior or equivalent outcomes for UKR in appropriately selected patients.
• Survival Rates: Modern UKR designs demonstrate 10-year survival rates comparable to TKA (typically 90-95%), particularly when strict patient selection criteria are applied. Long-term (15-20 year) survival rates are also encouraging, though revision rates for UKR tend to be slightly higher than TKA, primarily due to progression of OA in the uninvolved compartments or aseptic loosening. However, revision of a UKR to TKA is generally less complex and yields good results compared to revision of a failed TKA.
• Complications: While UKR typically has lower rates of major complications (e.g., DVT, PE, infection) compared to TKA due to less surgical trauma, specific complications like bearing dislocation (for mobile-bearing designs) or progression of arthritis are more unique to UKR.

Fixed-Bearing vs. Mobile-Bearing Designs

• Fixed-bearing UKR: Simpler technically, more forgiving for ligamentous balance, and generally robust. Long-term results are excellent with established designs.
• Mobile-bearing UKR: Designed to offer more physiological motion and theoretically reduce polyethylene wear. Requires meticulous surgical technique and precise ligamentous balance to prevent bearing dislocation. Meta-analyses and registry data show similar overall survival rates between fixed and mobile-bearing designs, though some studies suggest a slightly higher early revision rate for mobile-bearing due to dislocation, while others indicate potentially lower wear rates long-term. The choice often reflects surgeon preference and experience.

Robotic-Assisted UKR

• Accuracy: Multiple studies have demonstrated that robotic-assisted UKR achieves higher accuracy and precision in component positioning and limb alignment compared to conventional manual techniques, reducing outliers.
• Clinical Outcomes: While improved accuracy is clear, evidence for superior long-term clinical outcomes (e.g., survival, patient-reported outcomes) directly attributable to robotics is still evolving. Early studies suggest comparable or potentially improved short-to-mid-term outcomes and satisfaction. Robotic assistance may flatten the learning curve for surgeons new to UKR.

Current Guidelines and Debates

• Patient Selection: The consensus among major orthopedic societies (e.g., AAOS, British Orthopaedic Association) remains centered on rigorous patient selection, emphasizing isolated unicompartmental disease, intact cruciate ligaments, correctable deformity, and healthy patellofemoral joint.
• Age and BMI: The traditional absolute contraindications for age and BMI are softening. Younger, active patients can be considered, especially given the bone-preserving nature of UKR and the easier conversion to TKA if needed. Higher BMI patients (up to 35-40 kg/m²) are increasingly undergoing UKR, though with caution regarding potential increased risks.
• Lateral UKR: Lateral UKR remains less common than medial UKR due to its lower incidence of isolated lateral OA, higher technical demands, and historically slightly inferior outcomes. However, with appropriate patient selection and experienced surgeons, good results can be achieved.
• Learning Curve: UKR has a recognized steep learning curve. Surgeon experience significantly impacts outcomes, with higher volume surgeons achieving better results and lower revision rates.
• Future Directions: Research continues on improved implant materials, personalized kinematics, and expansion of indications (e.g., highly selected ACL-deficient patients with stable knees and specific implant designs).

In conclusion, UKR is a viable and often superior option for the appropriately selected patient with isolated unicompartmental knee osteoarthritis. Adherence to strict indications, meticulous surgical technique, and a comprehensive rehabilitation protocol are paramount for optimizing long-term success.