Knee Arthroplasty Unicondylar: Discover if Partial Knee Replacement is Right

Comprehensive Introduction and Patho-Epidemiology

Evolution of Unicondylar Knee Arthroplasty

The human knee is classically divided into three distinct biomechanical compartments: the medial compartment, the lateral compartment, and the patellofemoral joint. Unicondylar knee arthroplasty (UKA) is a highly specialized surgical intervention performed for isolated medial or lateral compartment osteoarthritis (OA). Historically, the reputation of UKA was marred by unpredictable survivorship and high early revision rates. These early failures were largely attributable to a triad of suboptimal variables: poor patient selection, imprecise surgical technique, and rudimentary component designs that failed to replicate native knee kinematics.

In the modern era of adult reconstruction, the paradigm has shifted dramatically. The key to a successful, durable outcome following UKA is now understood to be a meticulous "marriage" between these three variables. Advanced biomaterials, highly conforming mobile and fixed-bearing designs, and stringent adherence to specific kinematic alignment principles have resurrected UKA as a premier joint preservation arthroplasty. Unlike total knee arthroplasty (TKA), which relies on extensive soft tissue release and non-anatomic ligamentous balancing, UKA is fundamentally a resurfacing procedure that relies on the functional integrity of the native cruciate and collateral ligaments.

When executed correctly in an appropriately selected patient, UKA offers profound advantages over TKA. These include a smaller surgical incision, preservation of bone stock, reduced intraoperative blood loss, lower perioperative morbidity, and a faster return to baseline functional activities. Furthermore, kinematic studies consistently demonstrate that patients with a successful UKA exhibit gait patterns and proprioceptive feedback that more closely mimic the native, non-arthritic knee compared to their TKA counterparts.

Pathogenesis of Medial Unicompartmental Osteoarthritis

Medial unicompartmental osteoarthritis, when presenting with intact cruciate ligaments and a functionally normal medial collateral ligament (MCL), results in a highly recognizable, stereotypic pattern of articular wear. This specific pathological entity is often referred to as "anteromedial osteoarthritis." The cartilage and subchondral bone erosions are classically found on the anteromedial tibial plateau and the corresponding distal weight-bearing surface of the medial femoral condyle. This represents a distinct pattern of "extension disease," where the maximal articular contact and subsequent abrasive wear occur when the knee is in full extension or early flexion.

Crucially, in true anteromedial OA, the erosive changes rarely extend to the posterior quarter of the tibial plateau and virtually never reach the posterior joint margin. The articular cartilage is predictably preserved on the flexion surface of the femur (the posterior condyle) and the posterior aspect of the tibial plateau. This preservation is a direct biomechanical consequence of the intact anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). The intact ligaments maintain normal femoral "roll-back" during deep flexion, shifting the contact point posteriorly onto pristine, un-eroded cartilage.

As the anterior cartilage is progressively destroyed, the joint space narrows anteriorly, leading to a varus deformity. To expose eburnated subchondral bone on both the tibia and femur, approximately 5 mm of combined articular cartilage must be lost. Biomechanically, this 5 mm loss typically translates to a 5-degree varus deformity in the coronal plane. For every additional millimeter of structural bone loss that occurs after the cartilage is completely denuded, the varus deformity will predictably increase by 1 degree.

Natural History and Cruciate Ligament Failure

The natural history of anteromedial OA is intrinsically tied to the functional status of the ACL. Degenerative failure of the ACL is often the catastrophic sentinel event that causes isolated anteromedial OA to transition into diffuse posteromedial OA. Once the ACL becomes incompetent, the normal kinematic roll-back is lost. The femur subluxates anteriorly on the tibia, causing the posterior weight-bearing axis to shift. This results in posterior tibial subluxation and subsequent structural shortening and contracture of the MCL.

When this transition occurs, the varus deformity is no longer correctable in flexion, as the posterior cartilage is rapidly worn away. The ACL itself progresses through well-documented stages of degenerative failure in the arthritic knee. It begins as a normal ligament, followed by a loss of its protective synovial covering (usually starting distally). This exposes the collagenous tendon to the hostile, inflammatory intra-articular environment, leading to longitudinal splits, stretching, loss of tensile strength, and finally, complete rupture with eventual resorption of the ligamentous remnant.

Chronic synovitis, a hallmark of progressive OA, causes nutritional insufficiency to the ACL, placing it at high risk for spontaneous rupture. Furthermore, the development of intercondylar osteophytes from the arthritic process physically impinges on the ACL within the notch, mechanically abrading the ligament and accelerating its degenerative failure. Therefore, identifying the exact stage of the disease process is paramount; once the ACL is compromised, the patient is no longer a candidate for a standard unicompartmental resurfacing and must be transitioned to a TKA pathway.

Detailed Surgical Anatomy and Biomechanics

Compartmental Anatomy and Ligamentous Tension

A profound understanding of knee anatomy and its dynamic biomechanics is non-negotiable for the orthopedic surgeon performing UKA. The three distinct compartments of the knee—the lateral compartment (lateral tibial plateau and lateral femoral condyle), the medial compartment (medial tibial plateau and medial femoral condyle), and the patellofemoral joint—each possess unique kinematic profiles. In the normal, non-arthritic knee, the majority of the capsuloligamentous structures are at their maximal resting, unstretched lengths when the knee is in full extension, providing static coronal and sagittal stability.

As the knee initiates flexion, the dynamic tension of these structures alters significantly. At approximately 20 to 30 degrees of flexion, the posterior capsule and the lateral collateral ligament (LCL) begin to slacken. This physiological relaxation allows for a functional gap under tension, which is critical for normal axial rotation and roll-back. Further gapping or distraction of the joint is strongly resisted by the central pivot—the ACL and PCL—as well as the robust medial collateral ligament (MCL), which remains the primary restraint to valgus stress.

At 90 degrees of deep flexion, the asymmetrical nature of the knee's compartments becomes evident. Under applied tension, the lateral compartment will distract approximately 7 mm, reflecting the inherent laxity and mobility of the lateral meniscus and LCL. In stark contrast, the medial compartment maintains a highly constrained, constant 2-mm gap. This is because the cruciate ligaments and the MCL exert an isometric, tethering effect on the medial compartment throughout the entire arc of motion, ensuring medial stability while allowing the lateral side to pivot around it.

Load Distribution and the Mechanics of Varus Deformity

The mechanical axis of the lower extremity dictates the load distribution across the tibial plateau. In a normal, neutrally aligned knee during a single-leg stance, the load vector passes slightly medial to the center of the joint. Consequently, the load across the medial compartment is approximately 70% of the total body weight force, while the lateral compartment absorbs the remaining 30%. This inherent physiological bias explains why medial compartment OA is exponentially more common than lateral compartment OA.

When a varus deformity develops—typically initiated by the loss of medial articular cartilage—this load distribution becomes severely skewed. A seemingly mild varus deformity of just 4 to 6 degrees shifts the mechanical axis further medially, increasing the load across the medial compartment to an overwhelming 90%. This massive increase in focal contact stress accelerates subchondral bone failure, microfractures, and the rapid progression of eburnation, creating a vicious cycle of progressive varus collapse.

In the presence of this varus deformity, the medial soft tissues adapt. The posterior capsule structurally shortens over time. However, in pure unicompartmental disease, when the capsule is relaxed at 20 degrees of flexion, the knee can still be corrected manually to its pre-diseased, neutral alignment. At 90 degrees of flexion, the knee will often correct spontaneously as the preserved cartilage on the flexion surface of the femur comes into contact with the preserved posterior tibia, tensioning the MCL back to its normal physiological length.

Implications for Surgical Balancing

Because the pathology of anteromedial OA is primarily a disease of cartilage and bone loss rather than primary ligamentous contracture, the surgical philosophy of UKA differs fundamentally from TKA. Unlike tricompartmental disease, unicompartmental disease should never require any ligamentous release during arthroplasty. The goal of UKA is not to release the MCL to balance the knee, but rather to replace the exact volume of lost cartilage and bone with prosthetic components, thereby restoring the native tension of the intact MCL.

If a surgeon finds themselves tempted to perform a medial release during a UKA, it is a clear indication that either the tibial resection was inadequate, the selected bearing is too thick, or the patient has progressed to tricompartmental disease with fixed soft-tissue contractures that contraindicate a partial knee replacement. The arthroplasty must respect the isometric nature of the medial compartment; overstuffing the medial joint space will inevitably lead to lateral compartment overload, rapid progression of lateral OA, and early clinical failure.

Exhaustive Indications and Contraindications

Patient History and Physical Examination

The quintessential symptom of unicompartmental OA is mechanical joint pain. This pain is typically localized along the medial joint line, though patients may occasionally report diffuse anterior knee pain, making precise localization somewhat unreliable based on history alone. The pain is characteristically exacerbated by weight-bearing activities such as standing, walking, and particularly descending stairs, and is reliably relieved by sitting or lying down. Rest pain or severe night pain should raise clinical suspicion for alternative pathologies, such as infection, inflammatory arthropathy, or complex regional pain syndrome.

On physical examination, a varus deformity of 5 to 15 degrees is typically observed while the patient is standing. Crucially, this deformity must correct passively. When the knee is brought to 90 degrees of flexion, or when a valgus stress is applied at 20 degrees of flexion, the alignment should normalize, indicating that the MCL is not structurally contracted. A mild flexion contracture (less than 15 degrees) is often present, frequently accompanied by a small to moderate joint effusion and localized synovial swelling along the medial joint line.

Assessing ligamentous stability in the arthritic knee can be challenging. The Lachman’s test, pivot shift, and anterior drawer tests are often difficult to interpret due to osteophytic impingement, pain guarding, and altered joint kinematics. However, it is imperative that gross anterior-posterior and varus-valgus stability be present. If a definitive endpoint cannot be felt during a Lachman's test, or if the tibia subluxates anteriorly on the lateral radiograph, ACL deficiency must be assumed until proven otherwise intraoperatively.

The Oxford Criteria for Patient Selection

The decision to proceed with UKA is ultimately confirmed in the operating room, but it relies heavily on strict preoperative criteria. The most widely accepted guidelines are the Oxford criteria, which are divided into physical, radiographic, and intraoperative signs. Physically, the patient must have pain severe enough to justify the risks of joint replacement surgery, and they must possess a flexion deformity of less than 15 degrees. A contracture greater than 15 degrees indicates posterior capsular contracture that cannot be corrected without extensive soft tissue release, contraindicating UKA.

Radiographically, the Oxford criteria demand evidence of full-thickness cartilage loss with eburnated, bone-on-bone contact in the medial compartment on weight-bearing views. Simultaneously, there must be full-thickness cartilage preservation in the lateral compartment, an intact articular surface at the posterior aspect of the medial tibial plateau, and a manually correctable varus deformity. These radiographic signs confirm the diagnosis of isolated anteromedial OA and verify that the disease has not progressed to the posterior plateau or the lateral side.

Intraoperatively, the final decision is made. The surgeon must visually confirm the presence of an intact, functional ACL. A frayed or attenuated ACL may be acceptable, but a completely ruptured or non-functional ACL is an absolute contraindication for a mobile-bearing UKA. Furthermore, the central weight-bearing articular cartilage of the lateral compartment must be inspected and deemed satisfactory. Minor fibrillation is acceptable, but exposed subchondral bone laterally necessitates immediate conversion to a TKA.

Modern vs. Traditional Contraindications

The landscape of UKA contraindications has evolved significantly over the past two decades. Traditional contraindications historically included inflammatory arthritis (e.g., rheumatoid arthritis), a flexion contracture of 5 or more degrees, a preoperative arc of motion of less than 90 degrees, an angular deformity of more than 15 degrees, significant cartilaginous damage in the opposite compartment, ACL deficiency, a Body Mass Index (BMI) higher than 32, and exposed subchondral bone beneath the patella.

However, extensive long-term registry data and high-quality prospective studies have forced a re-evaluation of these rules. Today, criteria that are no longer considered absolute contraindications include advanced age, high activity level, heavy body weight (obesity), asymptomatic patellofemoral arthritis, the presence of a degenerative lateral meniscus, and chondrocalcinosis. Studies have shown that patients with asymptomatic Grade IV patellofemoral changes do just as well with a medial UKA as those with pristine patellofemoral joints, provided the primary pain generator is the medial compartment.

Pre-Operative Planning, Templating, and Patient Positioning

Essential Imaging Modalities

Meticulous preoperative imaging is the cornerstone of successful UKA planning. Plain radiographs must include a weight-bearing anteroposterior (AP), a true lateral, a Rosenberg (45-degree flexion weight-bearing posteroanterior) view, and a Merchant or sunrise patellar view. These images are critical in determining whether the patient is an appropriate candidate. If only medial compartment arthritis—evidenced by joint space narrowing, subchondral sclerosis, cystic changes, and osteophyte formation—is appreciated on the standard AP view, a valgus stress view is absolutely mandatory.

The valgus stress radiograph is arguably the most important image in the UKA workup. It serves two critical functions: first, it demonstrates the normal thickness of the cartilage in the lateral compartment by opening the lateral joint space; second, it proves whether the medial varus deformity is passively correctable. On a proper valgus stress view, the lateral compartment should gap appropriately, and the lateral cartilage space should not measure less than 5 mm (representing the sum of the thickness of normal femoral and tibial cartilage). Conversely, the medial compartment should gap at least 5 mm, representing the sum of the articular cartilage lost.

Incomplete loss of the medial joint space on a standard AP must be investigated further with a varus stress view to confirm complete joint space loss. If the joint space loss is not full-thickness (i.e., not bone-on-bone), UKA should generally be avoided, as the outcomes for partial cartilage loss are significantly inferior, and other sources of pain (e.g., meniscal pathology, radiculopathy) must be sought.

Assessing the ACL via Radiography

The lateral radiograph is a highly reliable, albeit indirect, indicator of the functional integrity of the ACL. Because anteromedial OA features preserved posterior cartilage due to normal kinematic roll-back, the lateral film can reveal the extent of posterior wear. For accurate assessment, the femoral condyles and the medial and lateral tibial plateaus should appear perfectly superimposed.

Clinical & Radiographic Imaging Archive