Mobile Bearing Unicondylar Knee Replacement

Archival Presentation Notice: This article modernizes the multimedia presentation “Mobile-Bearing Unicompartmental Knee Arthroplasty” originally presented by Adolph V. Lombardi, Jr., MD, FACS (Joint Implant Surgeons, Mount Carmel Health System, The Ohio State University). The historical Flash lecture has been reconstructed into the synchronized video slide presentation above, with the original file preserved in the digital archives (Mobile_Bearing_Unicondylar_Replacement_Lombardi.swf).

1. High-Yield Summary

Mobile-bearing unicompartmental knee arthroplasty (UKA) represents a kinematically restorative, bone-preserving surgical option for end-stage isolated medial compartment osteoarthritis. By uncoupling articular conformity from rotational constraint, the mobile-bearing design provides full spherical contact throughout the entire arc of motion while permitting unconstrained planar translation of the ultra-high-molecular-weight polyethylene (UHMWPE) meniscal bearing across a polished flat cobalt-chromium tibial baseplate.

• Primary Clinical Indication: Isolated anteromedial osteoarthritis (AMOA) characterized by full-thickness cartilage erosion on both the medial femoral condyle and the anterior medial tibial plateau, functionally and structurally intact cruciate ligaments (specifically an intact anterior cruciate ligament), a passively correctable intra-articular varus deformity, and fully preserved lateral and patellofemoral compartments.
• Biomechanical Advantage: The fully conforming spherical articulation between the femoral condyle and the meniscal bearing reduces peak contact stress to 3 to 5 MPa (well below the 15 to 20 MPa yield strength of polyethylene), virtually eliminating volumetric wear and fatigue delamination while avoiding the transmission of shear stresses to the implant-bone interface.
• Critical Procedural Move: Strict preservation of the deep and superficial medial collateral ligament (MCL) fibers without surgical release, combined with precise flexion-extension gap balancing and deliberate retention of 2 to 4 degrees of constitutional varus alignment to avoid catastrophic lateral compartment overloading.
• Catastrophic Errors to Avoid: Overstuffing the medial flexion gap, plunging the sagittal saw beyond the posterior tibial cortex into the popliteal neurovascular bundle, creating a stress riser by notching the posterior cortex, and leaving residual posterior or medial osteophytes that cause soft-tissue tenting, bearing impingement, or meniscal bearing dislocation.

2. Indications and Patient Selection

Patient selection is the primary determinant of long-term survivorship in mobile-bearing unicompartmental knee arthroplasty. Unlike total knee arthroplasty, which replaces all articulating surfaces, UKA relies entirely upon the health and stability of the retained native compartments and soft-tissue structures.

Absolute Indications

• Isolated Anteromedial Osteoarthritis (AMOA): Full-thickness cartilage loss (“bone-on-bone” contact) affecting both the medial femoral condyle and the anterior two-thirds of the medial tibial plateau (Ahlbäck grade III/IV or Kellgren-Lawrence grade 4).
• Spontaneous Osteonecrosis of the Knee (SONK / Ahlbäck Disease): Medial femoral condylar osteonecrosis with subchondral collapse less than 5 mm and involving less than 50% of the condylar width, with intact cruciate ligaments and absence of secondary lateral degeneration.
• Intact Cruciate Ligaments: A functionally and anatomically intact anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). An intact ACL prevents abnormal anterior and posterior tibial translation, ensuring that contact loads remain centered over the spherical bearing throughout flexion.
• Passively Correctable Varus Deformity: The intra-articular varus deformity must correct fully to neutral alignment upon gentle manual valgus stress in extension. This confirms that the deformity is driven by articular cartilage loss rather than fixed collateral ligament contracture.
• Pristine Lateral Compartment: Full-thickness articular cartilage and intact lateral meniscus, demonstrated radiographically by normal joint space height (>5 mm) on stress views.

Relative Indications

• Patellofemoral Joint Changes: Asymptomatic patellofemoral osteoarthritis, anterior osteophytes, or medial facet remodeling are fully compatible with mobile-bearing UKA. Only lateral facet bone-on-bone eburnation with patellar subluxation or severe anterior knee pain limiting stair ambulation warrants exclusion.
• Chronological Age and Activity Level: Mobile-bearing UKA is highly effective across the age continuum. Younger, active patients benefit from physiological joint kinematics, bone preservation, and rapid rehabilitation; older patients benefit from reduced systemic morbidity, minimal blood loss, and outpatient protocol safety.
• Elevated Body Mass Index (BMI): Contemporary multicenter studies and registry data have dismantled historical weight cutoffs. Patients with BMI exceeding 35 kg/m2 demonstrate long-term implant survivorship equivalent to non-obese cohorts when anatomical alignment and soft-tissue balancing principles are strictly respected.
• Mild Flexion Contracture: Preoperative flexion contractures up to 10 to 15 degrees routinely correct intraoperatively following the excision of anterior femoral and intercondylar notch osteophytes.

Strict Contraindications

• Deficient or Attenuated Anterior Cruciate Ligament: Complete absence or structural compromise of the ACL permits posterior subluxation of the tibia during knee flexion, shifting load onto the posterior tibial rim and causing rapid bearing dislocation or catastrophic tibial tray loosening.
• Fixed, Uncorrectable Deformity: Fixed varus deformity exceeding 10 to 15 degrees or fixed flexion contracture exceeding 15 degrees indicating fixed capsular and collateral contracture.
• Lateral Compartment Degeneration: Joint space narrowing, chondromalacia, full-thickness cartilage defects, or prior lateral meniscectomy in the lateral compartment.
• Inflammatory Arthritis: Rheumatoid arthritis, psoriatic arthritis, lupus, or other autoimmune inflammatory arthropathies, where pan-articular synovial inflammation leads to progressive multi-compartmental destruction.
• Incompetent Medial Collateral Ligament: Disruption, attenuation, or prior surgical division of the deep or superficial MCL, which precludes dynamic soft-tissue containment of the mobile bearing.
• Active Infection: Active or recent deep articular knee sepsis, severe periarticular skin compromise, or active systemic bacteremia.

3. Biomechanical Principles: Mobile-Bearing vs. Fixed-Bearing

The biomechanical philosophy of unicompartmental arthroplasty addresses the fundamental trade-off between articular conformity, contact stress, and kinematic constraint.

Contact Stress & Spherical Conformity

Fixed-bearing unicompartmental designs face a mechanical paradox. A flat polyethylene insert permits unconstrained multi-axis rotation and condylar translation, but produces line or point contact that generates peak contact pressures exceeding 20 to 40 MPa. Because these stresses surpass the compressive yield strength of ultra-high-molecular-weight polyethylene (15 to 20 MPa), fixed-bearing implants are prone to subsurface shear fatigue, delamination, and pitting wear. Conversely, if a fixed-bearing insert is dished to increase conformity and lower contact stress, it imposes rotational constraint. This constraint transfers cyclic shear and torsional moments directly to the bone-implant or cement-bone interface, predisposing the construct to early aseptic loosening.

Mobile-bearing arthroplasty resolves this paradox by decoupling kinematic freedom from articular conformity. The femoral component features a single spherical radius of curvature throughout the functional arc of flexion. The superior surface of the meniscal bearing is fully spherical and identical in radius, creating congruent 1:1 area contact. As a result, contact stress is distributed over a broad surface area, maintaining peak pressures between 3 and 5 MPa throughout the entire range of motion.

Kinematics & Meniscal Bearing Translation

The inferior surface of the mobile bearing is flat, articulating with a mirror-polished flat cobalt-chromium tibial baseplate. This planar interface allows unrestricted anteroposterior translation and axial rotation:

• Physiological Femoral Rollback: During knee flexion, the medial femoral condyle translates posteriorly. The mobile meniscal bearing translates posteriorly with the condyle (translating 6 to 10 mm), guided by the spherical femoral contour and stabilized by the intact soft-tissue envelope.
• Absence of Constraint: Because the bearing translates freely on the tibial plate, no torsional or shear forces are transmitted to the tibial baseplate or its fixation interface, protecting the prosthesis from loosening.
• Ligamentous Kinematic Guidance: The intact cruciate and collateral ligaments dictate natural joint motion, preserving normal screw-home rotation and proprioception.

Wear Mechanics & Polyethylene Longevity

Mobile-bearing UKA operates with two distinct articulating surfaces: the upper spherical interface and the lower planar interface. In vitro knee simulator investigations and long-term retrieval analyses demonstrate that total volumetric wear in mobile-bearing UKA is exceptionally low (typically less than 2 to 3 mm3 per year), translating to a linear penetration rate below 0.01 to 0.03 mm annually. Furthermore, because the mobile bearing is not mechanically locked into the tibial tray, the back-side fretting wear and osteolysis commonly seen in modular fixed-bearing constructs are entirely avoided.

4. Preoperative Planning & Radiographic Evaluation

Systematic preoperative evaluation confirms eligibility for unicompartmental reconstruction, predicts component sizing, and maps periarticular osteophytes.

Radiographic Imaging & Stress Radiography

• Weight-Bearing Anteroposterior (AP) and Lateral Radiographs: True AP view demonstrates medial joint space collapse. The strict lateral radiograph (taken at 30 degrees of flexion) must demonstrate that cartilage loss is restricted to the anterior two-thirds of the medial tibial plateau. Full-thickness cartilage must be preserved on the posterior tibial plateau; posterior erosion signifies functional ACL deficiency.
• Rosenberg 45-Degree Flexion Posteroanterior View: Visualizes posterior condylar cartilage loss that may be hidden on standard extension views.
• Merchant or Skyline Patellofemoral View: Evaluates patellofemoral congruence and excludes severe lateral facet erosion or patellar subluxation.
• Valgus and Varus Stress Radiography: A manual or instrumented valgus stress radiograph is the most critical diagnostic tool. It verifies that the medial joint deformity opens passively and confirms that the lateral compartment possesses full cartilage thickness (>5 mm) without joint space narrowing.
• Full-Length Standing Hip-to-Ankle Alignment Radiograph: Establishes the mechanical axis of the lower extremity, calculates native joint line obliquity, and confirms that constitutional varus does not exceed 10 degrees.

Preoperative Component Sizing & Osteophyte Assessment

Digital templating is performed to estimate tibial and femoral component dimensions:

• Tibial Baseplate Sizing: Sized on the lateral radiograph to match the anteroposterior dimension of the medial tibial plateau from anterior to posterior cortex. Anterior or posterior overhang must be avoided to prevent impingement, while medial under-coverage must not exceed 1 to 2 mm to prevent bone subsidence.
• Femoral Component Sizing: Sized to restore posterior condylar offset without overhanging laterally into the intercondylar notch or impinging upon the patella in deep flexion.
• Osteophyte Mapping: Extensive osteophytes along the medial condyle, intercondylar notch, and medial tibial margin must be identified. Preoperative mapping ensures their complete intraoperative removal, preventing false ligament tensioning.

Examination Under Anesthesia

Immediately prior to sterile draping, manual examination under anesthesia (EUA) provides the final checkpoint:

• Lachman and Anterior Drawer Maneuvers: Confirmation of a crisp, firm endpoint for the anterior cruciate ligament. Gross anterior translation or an absent endpoint mandates aborting UKA and converting to total knee arthroplasty.
• Passive Valgus Stress at 0 and 30 Degrees: The limb must effortlessly correct to neutral alignment. Inability to correct to neutral indicates fixed medial collateral ligament contracture.
• Terminal Extension and Flexion Range: Documentation of baseline range of motion and confirmation of the absence of fixed flexion contracture exceeding 15 degrees.

5. Patient Positioning & Surgical Corridors

Meticulous patient positioning and adherence to anatomical internervous corridors ensure unobstructed visualization while protecting surrounding neurovascular structures.

Limb Positioning & OR Setup

• Operating Table and Patient Position: The patient is positioned supine on a standard radiolucent operating table. A pneumatic tourniquet is applied to the proximal thigh of the operative limb.
• Leg Positioning Alternatives (Leg-Hanging Position): The foot of the operating table is dropped 90 degrees, allowing the operative leg to hang freely with a lateral thigh post. Gravity assists in opening the medial joint compartment in flexion.
• Leg Positioning Alternatives (Supine Bolster Position): The limb remains on a flat table supported by a lateral thigh support and a sterile foot bumper or adjustable leg holder. Validated in prospective trials, this configuration achieves equivalent component alignment and gap balancing while reducing table manipulation and operating room personnel strain.
• Range of Motion Arc: The limb must be able to cycle freely from full extension (0 degrees) to at least 110 to 120 degrees of flexion throughout the procedure.

Surgical Approach & Internervous Corridors

• Incision: A 6 to 8 cm longitudinal midline or anteromedial skin incision is performed, beginning at the superior pole of the patella and extending distally to approximately 1 cm distal to the medial tibial joint line.
• Internervous Plane: The approach exploits the true intermuscular and internervous boundary medial to the quadriceps mechanism. The vastus medialis is innervated proximally by the femoral nerve, and no major motor nerves cross this corridor.
• Medial Parapatellar Arthrotomy: The capsule is incised longitudinally along the medial border of the patella, extending into the distal fibers of the vastus medialis tendon. The patella is subluxated laterally but is never everted, preserving the natural extensor mechanism tension and avoiding postoperative patellar tendon strain.
• Tibial Exposure: The anterior horn of the medial meniscus is excised. The coronary ligament is elevated conservatively along the anterior tibial rim, taking extreme care to preserve the insertion of the deep medial collateral ligament.

Neurovascular Structures at Risk & Danger Zones

• Popliteal Neurovascular Bundle: The popliteal artery and tibial nerve lie directly posterior to the central and posterior capsule of the knee. Plunging an oscillating saw blade through the posterior tibial cortex during the horizontal resection is the primary hazard. Protection: A specialized curved posterior retractor must be placed subperiosteally behind the posterior tibial cortex before initiating the horizontal cut.
• Medial Collateral Ligament Complex: The deep and superficial MCL fibers run immediately adjacent to the medial margin of the tibial resection. Skiving the saw blade medially or overzealous subperiosteal stripping destabilizes the knee, alters gap balance, and guarantees meniscal bearing dislocation. Protection: Keep saw cuts inside the medial cortical margin; never release the MCL to balance gaps.
• Infrapatellar Branch of the Saphenous Nerve (IPBSN): Crosses the anterior proximal tibia obliquely. Transverse or lateral surgical extensions risk nerve laceration and painful neuroma. Protection: Maintain a strict longitudinal incision and avoid forceful lateral retraction on the subcutaneous tissues.
• Anterior Cruciate Ligament Footprint: The tibial insertion of the ACL lies adjacent to the medial tibial spine. An errant sagittal saw trajectory angled toward the intercondylar eminence will compromise stability. Protection: Align the sagittal cut strictly parallel to the tibial mechanical axis and medial to the tibial spine.

6. Surgical Technique & Critical Procedural Moves

The operative technique requires an exacting sequence of anatomical cuts, milling, and balance assessments to ensure bearing stability and physiological kinematics.

Step 1: Exposure & Cruciate Ligament Inspection

• Incise the skin and capsule through a minimally invasive medial parapatellar arthrotomy; place Hohmann retractors around the medial condyle and tibial plateau.
• Excise the medial meniscus completely back to its posterior root attachment, and resect the infrapatellar fat pad sufficiently to visualize the intercondylar notch.
• Inspect the anterior cruciate ligament directly using a blunt nerve hook. The surgeon must verify structural tension and normal fibrillar architecture throughout its length. Direct visual inspection of the lateral compartment and patellofemoral joint must confirm pristine articular cartilage.
• Debride all medial, anterior, and intercondylar osteophytes using an osteotome and rongeur. Thorough osteophyte excision decompresses the medial soft-tissue envelope, restoring the true resting tension of the MCL.

Step 2: Sagittal & Horizontal Tibial Resection

• Apply the extramedullary tibial guide aligned parallel to the mechanical axis of the tibia in the coronal plane, setting the sagittal slope to match the patient’s native medial tibial slope (typically 5 to 7 degrees of posterior slope).
• Perform the vertical (sagittal) cut: Position the sagittal saw blade immediately medial to the apex of the medial tibial spine, aimed toward the anterior superior iliac spine (ASIS). Make the cut perpendicular to the horizontal plane, cutting down to the planned level of horizontal resection.
• Perform the horizontal resection: Place a curved protective retractor behind the posterior tibial cortex. Set the cutting guide to resect only 2 to 3 mm of subchondral bone from the deepest point of the eroded medial plateau. Make the horizontal cut with an oscillating saw, stopping precisely at the sagittal cut without notching the lateral tibial spine.
• Remove the resected tibial bone wafer and examine: Confirm uniform thickness (2 to 4 mm), absence of posterior cortical steps, and clean bone margins.

Step 3: Femoral Milling & Spigot Referencing

• Establish intramedullary femoral alignment: Drill an entry hole in the medial femoral condyle 1 cm anterior to the origin of the posterior cruciate ligament, aiming along the anatomical axis of the femur, and advance the intramedullary rod.
• Position the femoral spigot: Place the femoral sizing guide against the resected tibial surface and posterior femoral condyle to reference the flexion gap and determine spigot position.
• Mill the spherical condyle: Lock the spherical milling cutter onto the spigot. Sequentially mill the distal and posterior condyle until the subchondral bone is contoured to match the spherical geometry of the femoral implant.
• Prepare fixation peg holes and chamfers: Drill the central femoral peg hole and complete the posterior chamfer cut using the dedicated template, irrigating thoroughly.

Step 4: Component Trialling & Gap Balancing

• Seat trial tibial baseplate and trial femoral component into their respective bone beds.
• Measure gaps using dedicated feeler gauges or trial meniscal bearings: in 90 degrees flexion, insert the feeler gauge, verifying that it slides with light resistance without lifting the femoral trial; in 20 degrees extension, insert the feeler gauge to confirm that resistance identically matches that of the flexion gap.
• Balance gap discrepancies: If the flexion gap is tighter than the extension gap, increase tibial slope or mill additional posterior femur; if the extension gap is tighter than the flexion gap, mill additional distal femur.

Step 5: Fixation Strategy: Cemented vs. Cementless Hydroxyapatite

• Cemented Fixation Technique: Perform high-pressure pulsatile lavage of both bone beds to eliminate lipids and bone slurry, followed by sponge drying. Pressurize doughy PMMA bone cement into the trabecular bone of the tibia and femur. Impact the tibial baseplate first, verifying full posterior and medial cortical seating without anterior lift-off. Impact the femoral component firmly into its peg hole. Meticulously excise all extruded cement circumferentially, paying special attention to the posterior capsule and intercondylar notch with a curved curette. Retained cement beads cause bearing impingement or dislocation.
• Cementless Hydroxyapatite Fixation Technique: Dual-coated implants (titanium porous plasma spray with a 50-micrometer hydroxyapatite layer) allow biological osteointegration. Impact the cementless tibial component with direct press-fit against flat, uninjured subchondral bone. Impact the cementless femoral component, verifying rigid initial scratch-fit fixation. Cementless fixation eliminates the risks of extruded cement, shortens surgical duration, and avoids the physiological radiolucencies seen under cemented tibial trays.

Step 6: Meniscal Bearing Insertion & Tracking Assessment

• Select the definitive UHMWPE meniscal bearing corresponding to the verified trial gauge thickness (typically 3 mm or 4 mm).
• Slide the bearing onto the polished tibial plate under the spherical femoral condyle with the knee in mid-flexion.
• Cycle the knee through full extension to deep flexion (0 to 120 degrees): verify that the bearing translates smoothly without edge lift-off, tilting, or anterior subluxation.
• Perform the “dislocation test” at 90 degrees of flexion: attempt to dislodge the bearing with a blunt probe; intact soft-tissue tension must retain the bearing securely.

Critical Move: Bearing Security & Prevention of Over-Correction

The definitive point of no return is selecting bearing thickness and securing mechanical alignment. The master surgeon must deliberately under-correct the knee, preserving 2 to 4 degrees of constitutional varus alignment. Over-correcting into neutral or valgus alignment transfers loading across the tibial spines into the pristine lateral compartment, precipitating rapid lateral joint breakdown. Soft-tissue tension must be verified: the bearing must fit snugly without stretching the collateral ligaments or causing lateral compartment lift-off.

7. Surgical Pearls and Avoidable Pitfalls

Technical execution requires adherence to unwritten intraoperative nuances and vigilance against classic procedural pitfalls.

Pearls

• Complete Osteophyte Debridement: Always resect all medial, anterior, and notch osteophytes before taking any gap measurements. Retained osteophytes falsely elevate collateral ligament tension, tempting the surgeon to resect unnecessary bone.
• The 1-mm Rule for Sagittal Tibial Resection: Keep the sagittal saw strictly vertical and parallel to the mechanical axis, stopping immediately at the horizontal cut plane. Never saw distal to the horizontal cut, as any over-sawing creates a stress riser for tibial plateau fracture.
• Two-Finger Feeler Gauge Feel: The correct feeler gauge or trial bearing should insert with smooth, two-finger resistance. If a mallet or heavy thumb pressure is needed, the gap is overstuffed.
• Continuous Posterior Retractor Protection: Keep the curved posterior retractor seated behind the posterior tibial cortex throughout all horizontal saw cuts to maintain an impenetrable mechanical shield in front of the popliteal artery.
• Pulsatile Lavage Bone Bed Preparation: When using cemented fixation, perform aggressive pulsatile lavage to clear marrow fat and bone debris, followed by sponge drying to achieve a 2 mm cement mantle interdigitation.

Pitfalls

• Excessive Tibial Resection (>4 mm): Resecting deep into weak metaphyseal cancellous bone increases the risk of tibial tray subsidence and fracture. Rescue: Use a thicker bearing (5 to 7 mm) or convert to a cemented augmented baseplate if bone quality is severely compromised.
• Posterior Tibial Cortex Notching: Allowing the saw blade to kick back or penetrate the posterior cortex creates a stress riser. Rescue: Inspect the posterior wall; if an unstable fracture or breach is identified, enforce protected weight-bearing or revise to a stemmed tibial construct.
• Retained Extruded Cement: Loose cement extruded into the posterior capsule causes mechanical bearing impingement and dislocation in deep flexion. Rescue: Systematically sweep the posterior capsule with a curved Woodson or Cobb elevator under direct visualization before closing.
• Overstuffing the Medial Compartment: Selecting an oversized bearing forces the knee into valgus and opens the lateral compartment. Rescue: Downsize bearing thickness by 1 to 2 mm until the knee rests in 2 to 4 degrees of varus under manual evaluation.
• Impingement Against the Lateral Tibial Spine: Placing the tibial tray too close to the tibial spine or leaving bone ridges lateral to the vertical cut pinches the bearing. Rescue: Use a bone file or burr to plane down all bone ridges lateral to the vertical wall of the tray.

8. Postoperative Management & Rehabilitation

Postoperative care emphasizes immediate functional mobilization, multimodal pain control, and structured progression through physical rehabilitation phases.

Immediate Postoperative Protocol

• Weight-Bearing Status: Immediate full weight-bearing as tolerated (WBAT) on postoperative Day 0 with assistive devices (walker or crutches) for initial balance.
• Thromboprophylaxis: Low-dose aspirin (81 mg twice daily) for 4 to 6 weeks for standard-risk patients, or direct oral anticoagulants (DOACs) / low-molecular-weight heparin (LMWH) for patients with elevated venous thromboembolism risk factors.
• Multimodal Analgesia: Adductor canal block or intraoperative periarticular local infiltration analgesia (LIA), scheduled oral acetaminophen and NSAIDs/COX-2 inhibitors, and PRN short-acting oral opioids. Motor-sparing anesthesia promotes early quadriceps activation.
• Discharge Pathway: Same-day discharge or 23-hour observation pathway. Discharge criteria include independent ambulation, stair negotiation, and an active straight leg raise.

Phase-by-Phase Rehabilitation Protocol

9. Clinical Outcomes & Long-Term Implant Survivorship

Modern clinical evidence establishes mobile-bearing unicompartmental knee arthroplasty as a durable, highly efficacious treatment for anteromedial osteoarthritis, with long-term survivorship closely tied to surgical technique and case volume.

Long-Term Survivorship Metrics

• 10- and 15-Year Survivorship: High-volume single-center and designer series report 10-year cumulative survivorship between 94% and 98%, and 15-year survivorship reaching 88.6%. National registry data (such as the UK National Joint Registry and the Australian Orthopaedic Association National Joint Replacement Registry) report 10-year survivorship between 88% and 92%.
• Surgeon Volume Effect: Annual surgical volume is the single most powerful prognostic factor for implant survivorship. Surgeons performing more than 30 mobile-bearing UKAs per year achieve revision rates comparable to or better than total knee arthroplasty, whereas low-volume surgeons (<10 cases annually) experience higher revision rates, largely driven by lower thresholds to revise unexplained postoperative symptoms.
• Cemented vs. Cementless Fixation: Multicenter prospective randomized controlled trials and systematic reviews demonstrate equivalent or superior 10-year survivorship for cementless hydroxyapatite-coated mobile-bearing designs (>95%), with virtually zero loosening-related failures and a dramatic reduction in physiological radiolucent lines.

Functional Recovery & Patient-Reported Outcomes

• The TOPKAT Landmark Trial: The landmark TOPKAT randomized controlled trial demonstrated non-inferiority and equivalent functional improvement between partial and total knee arthroplasty at 5-year follow-up (mean Oxford Knee Score 37.0 for partial vs 36.3 for total), with partial knee arthroplasty demonstrating superior cost-effectiveness and higher quality-adjusted life year (QALY) gains.
• Range of Motion and Joint Kinematics: Patients undergoing mobile-bearing UKA achieve an average postoperative flexion of 125 to 130 degrees, compared to 110 to 115 degrees in matched TKA cohorts.
• “Forgotten Joint” Sensation: Validated Forgotten Joint Score (FJS-12) measurements demonstrate significantly higher scores in mobile UKA compared to TKA, reflecting preservation of native cruciate ligament proprioception and normal kinematics.

Revision Epidemiology & Modes of Conversion to TKA

• Modes of Failure: The leading indication for revision is progressive osteoarthritis in the lateral compartment (30% to 40% of revisions), followed by aseptic loosening (20% to 25%), meniscal bearing dislocation (10% to 15%), unexplained pain (10%), and deep infection (1% to 2%).
• Surgical Conversion to TKA: When revision is necessary, conversion of a mobile-bearing UKA to a total knee arthroplasty is technically straightforward. In the vast majority of cases, primary TKA implants can be utilized without the need for extensive metal augments, wedges, or stemmed revision components. Patient-reported outcomes following conversion of UKA to TKA are comparable to primary TKA and significantly superior to revision TKA following failed primary total knee replacement.

10. Complications & Failure Modes

Complications specific to mobile-bearing unicompartmental arthroplasty require early diagnostic recognition and standardized management protocols.

Bearing Dislocation & Subluxation

Meniscal bearing dislocation is a complication unique to mobile-bearing designs, occurring in approximately 0.5% to 2% of cases, typically within the first postoperative year:

• Etiology: Dislocation occurs when the flexion gap is looser than the extension gap, when retained posterior or medial osteophytes push the bearing, or when an inadvertent injury to the deep or superficial MCL creates coronal laxity.
• Clinical Presentation: Sudden onset of acute medial pain, inability to bear weight, mechanical locking, or a palpable anterior prominence.
• Management: Plain radiographs confirm bearing displacement. While closed reduction under sedation is occasionally feasible acutely, definitive management requires mini-open arthrotomy to resect causative osteophytes, verify MCL integrity, and exchange the bearing to an appropriately sized, thicker insert.

Progression of Lateral Compartment or Patellofemoral Disease

• Pathomechanics: Surgical over-correction of the limb into neutral or valgus alignment shifts the mechanical load vector across the tibial spine into the lateral compartment, causing rapid full-thickness cartilage erosion.
• Prevention: Maintain 2 to 4 degrees of residual anatomical varus; never overstuff the medial compartment with an excessively thick bearing.
• Management: Symptomatic lateral progression warrants conversion to total knee arthroplasty.

Aseptic Loosening & Tibial Impingement

• Aseptic Loosening: Most frequently affects the tibial baseplate in cemented designs due to inadequate bone preparation, sclerotic bone beds, or eccentric load distribution.
• Radiolucency Interpretation: Benign physiological radiolucencies (2 mm) accompanied by localized pain represent true mechanical failure. Hydroxyapatite-coated cementless designs virtually eliminate these radiolucent lines.
• Tibial Impingement: Residual bone spurs lateral to the vertical tibial cut impinge upon the mobile bearing, causing bearing wear, tilting, and shear forces that destabilize the tibial tray.

Periprosthetic Tibial Plateau Fracture

• Mechanism: Deep horizontal tibial resections (>4 mm) that enter weak metaphyseal bone, over-sawing past the junction of the horizontal and sagittal cuts (creating a stress riser), or forceful seating of the tibial tray keel into unyielding sclerotic bone.
• Prevention: Limit tibial resection to 2 to 3 mm; stop the saw blade precisely at the intersection of the vertical and horizontal cuts; prepare the keel slot cleanly.
• Management: Non-displaced fractures with well-fixed components can be managed with 6 to 8 weeks of protected non-weight-bearing; displaced fractures or compromised component fixation require conversion to TKA with a stemmed tibial baseplate.

11. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

12. References

• Beard DJ, Davies LJ, Cook JA et al. The clinical and cost-effectiveness of total versus partial knee replacement in patients with medial compartment osteoarthritis (TOPKAT): 5-year outcomes of a randomised controlled trial. Lancet. 2019. doi:10.1016/S0140-6736(19)31281-4 [Level I: Randomized Controlled Trial]
• Hamilton WG, Redfern RE, Klaassen MA et al. A Randomized Controlled Trial to Compare a Mobile-Bearing Cementless and Cemented Unicompartmental Knee: Results of an Investigational Device Exemption Study in the United States. J Arthroplasty. 2026. doi:10.1016/j.arth.2026.05.032 [Level I: Randomized Controlled Trial]
• Basgul C, Kurtz MA, Rainey JP et al. Cementless Unicompartmental Knee Arthroplasty: A Systematic Review of Survivorship and Revision Indications. J Arthroplasty. 2026. doi:10.1016/j.arth.2026.01.019 [Level I: Systematic Review / Meta-Analysis]
• Bonano JC, Barrett AA, Aggarwal VK et al. Supine Knee Positioning Does Not Interfere with Mobile-Bearing Unicompartmental Knee Arthroplasty Performance. J Knee Surg. 2023. doi:10.1055/s-0042-1748822 [Level I: Randomized Controlled Trial]
• Crawford DA, Rutledge-Jukes H, Alexander JS et al. 15-Year Follow-Up of Mobile Bearing Medial Unicompartmental Knee Arthroplasty. J Arthroplasty. 2023. doi:10.1016/j.arth.2023.01.024 [Level IV: Longitudinal Cohort Study]
• Melvin PR, Bertrand T, Berend K et al. The mobile bearing prosthesis: How to know when it’s right for my patient and tips for surgical success. J ISAKOS. 2024. doi:10.1016/j.jisako.2024.100330 [Level V: Surgical Technique & Clinical Review]
• Murray DW, Goodfellow JW, O’Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br. 1998. doi:10.1302/0301-620x.80b6.8177 [Level IV: Prospective Cohort Study]

13. Master Class Pro-Tip

Successful mobile-bearing unicompartmental knee arthroplasty hinges upon three inviolable surgical tenets: never release the medial collateral ligament, meticulously clear every peripheral and intercondylar osteophyte before evaluating gaps, and deliberately under-correct the limb into its pre-arthritic constitutional varus alignment (typically 2 to 4 degrees). Over-correction shifts mechanical loading vectors across the intercondylar eminence into the pristine lateral compartment, condemning the lateral cartilage to rapid failure within 18 months. When performing the sagittal tibial cut, orient the blade strictly medial to the medial tibial spine and parallel to the mechanical axis, ensuring the saw never notches or penetrates distal to the planned horizontal resection level to prevent a stress riser. Prior to bearing selection, confirm that the flexion and extension gaps are perfectly matched; a flexion gap that is even 1 mm looser than the extension gap predisposes the unconstrained meniscal bearing to spin-out and dislocation during deep flexion. Finally, whether employing cemented or hydroxyapatite-coated cementless fixation, thoroughly sweep the posterior capsule with a curved elevator to guarantee that no extruded cement bead or retained bone fragment remains to impinge upon the translation corridor of the mobile bearing.

Master Class Surgical Pearl: Mobile-Bearing Unicompartmental Knee Arthroplasty