With the recent increase in medial unicompartmental arthroplasty, this article reviews the design history, indications, results, and modern technique for the implantation of the Oxford mobile-bearing unicompartmental arthroplasty. The article also discusses how the indications for the Oxford differ from the historical indications for medial unicompartmental arthroplasty and supports this paradigm shift with review of the recent data. A detailed series of surgical pearls is also presented to help surgeons with the surgical nuances of the Oxford partial knee.
Key points
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Partial knee arthroplasty is growing in popularity, especially in active adults and those patients seeking less invasive surgery and rapid recovery.
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The mobile-bearing, fully congruent design of the Oxford knee has less wear than fixed-bearing designs.
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The long-term outcome data of the Oxford knee rival those of total knee arthroplasty without the inherent morbidity, mortality, and other risks. This success of the Oxford has begun to challenge the dogma surrounding total knee arthroplasty.
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The Oxford partial knee is ideal for outpatient procedures.
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The modern design and instrumentation allows for minimally invasive implantation.
Introduction
The past 2 decades have seen a resurgence of interest in unicompartmental knee arthroplasty (UKA) for the treatment of isolated knee arthritis. The resurgence of medial UKA is in part due to a movement toward less invasive techniques, quicker recovery, less overall morbidity, and preservation of normal knee kinematics. This growth in popularity, combined with recent reports of long-term success with medial UKA, has also begun to challenge the dogmatic belief that total knee arthroplasty (TKA) is the gold standard treatment of all knee arthrosis.
The Oxford mobile-bearing UKA (Biomet, Inc, Warsaw, IN) is an implant with a unique design that has a spherical femoral component, a polished flat tibial component, and a fully congruent polyethylene meniscal bearing. This design is in sharp contrast with most medial UKA devices that use an aspherical femoral component and fixed polyethylene tibial component. The traditional fixed-bearing design creates the opportunity for polyethylene wear secondary to high-contact stresses over low surface area. The fully congruent mobile-bearing design of the Oxford UKA has been shown to reduce polyethylene wear to 0.01 to 0.02 mm per year while maintaining more normal knee kinematics.
Introduction
The past 2 decades have seen a resurgence of interest in unicompartmental knee arthroplasty (UKA) for the treatment of isolated knee arthritis. The resurgence of medial UKA is in part due to a movement toward less invasive techniques, quicker recovery, less overall morbidity, and preservation of normal knee kinematics. This growth in popularity, combined with recent reports of long-term success with medial UKA, has also begun to challenge the dogmatic belief that total knee arthroplasty (TKA) is the gold standard treatment of all knee arthrosis.
The Oxford mobile-bearing UKA (Biomet, Inc, Warsaw, IN) is an implant with a unique design that has a spherical femoral component, a polished flat tibial component, and a fully congruent polyethylene meniscal bearing. This design is in sharp contrast with most medial UKA devices that use an aspherical femoral component and fixed polyethylene tibial component. The traditional fixed-bearing design creates the opportunity for polyethylene wear secondary to high-contact stresses over low surface area. The fully congruent mobile-bearing design of the Oxford UKA has been shown to reduce polyethylene wear to 0.01 to 0.02 mm per year while maintaining more normal knee kinematics.
Oxford design history and rationale
Goodfellow and colleagues developed the concept of the Oxford knee in the early 1970s. The initial premise of the design was to reduce polyethylene wear by reproducing the congruent nature of the native meniscus. This congruent design increased the contact area but greatly reduced the contact stresses. However, in order to achieve full congruency on both interfaces with a solid polyethylene meniscus, the femoral side needed to be spherical and the tibial side needed to be flat.
The implant was originally used as a bicompartmental knee replacement with poor survivorship. The survivorship of the Oxford knee was markedly improved once its use was limited to the medial compartment of ligamentously stable knees with bone-on-bone osteoarthritis. White and colleagues defined this distinct pattern of medial disease as anteromedial osteoarthritis and only observed this pattern in patients with an intact anterior cruciate ligament (ACL). Those patients with ACL deficiency tended to have a posteromedial wear patterns secondary to the chronic anterior subluxation of the medial tibia that occurs when the ACL is incompetent. This pattern of anteromedial osteoarthritis is now the primary indication for a medial Oxford UKA.
The Oxford UKA has undergone a series of modifications since the 1970s. However, the original concepts remain unchanged since its initial development ( Fig. 1 A). Femoral milling was developed in 1987 to accurately and safely prepare the medial femoral condyle and allow for minimally invasive implantation (see Fig. 1 B). The third phase of development in the late 1990s created additional femoral component sizes and a more anatomic meniscal bearing that improved tracking and diminished bearing impingement and rotation (see Fig. 1 C). This phase III design also incorporated anatomic femoral sizes and a novel instrumentation platform specifically designed for minimally invasive implantation.
The most recent phase of Oxford development started in 2009 and focused on improving the reliability of the instrumentation, eliminating impingement of the meniscal bearing, and slight modifications to the femoral design (see Fig. 1 D). The new Microplasty Instrumentation (Biomet, Inc, Warsaw, IN) of the Oxford UKA uses an intramedullary reference for preparing the femur and a reproducible stylus to create a more consistent tibial resection. The new twin-peg femoral design maintains the same spherical design concept but has an additional peg for rotational stability; a longer radius of curvature to maintain bearing congruency in high flexion angles; and smoother, rounded edges to reduce soft tissue irritation and impingement (see Fig. 1 D).
Indications
The typical radiographic evaluation of an Oxford UKA candidate is seen in Fig. 2 . It is important to notice that, in this case example, the knee corrects to a normal valgus alignment with valgus stress and that the lateral joint space is maintained. Fig. 3 shows patients who are not candidates for UKA based on the failure of the valgus stress film and the presence of a posteromedial wear pattern in ACL deficiency.
The indications for medial Oxford UKA as defined by Goodfellow and colleagues are:
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Bone-on-bone anteromedial osteoarthritic wear pattern
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Ligamentously normal knee with an intact ACL
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Correctable varus deformity
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Well-maintained, normal lateral joint space on valgus stress view radiograph (see Fig. 2 E; Fig. 4 )
Fig. 4
The valgus stress radiograph is performed by aiming the x-ray beam with 10° of tilt at the joint line of the knee flexed to 20° over a bolster. Light valgus pressure is applied using lead gloves.
The traditional Kozinn and colleagues inclusion criteria are not considered the current indications for the Oxford UKA. Contrary to popular dogma, mild to moderate patellofemoral disease is not considered a contraindication unless there is severe lateral patellar facet wear and grooving of the lateral trochlear ridge. There has been no study that links mild to moderate patellofemoral disease to failure of UKA or a predominating reason for conversion to TKA. There are multiple reports that support ignoring the patellofemoral disease in the presence of anteromedial osteoarthritis of the tibiofemoral joint.
Although all surgeons agree that knee replacement surgery is reserved for elderly patients with end-stage osteoarthritis who have failed conservative measures, there is currently no limitation with respect to patient age as long as the patient has bone-on-bone anteromedial disease and meets the inclusion criteria. Obesity was once seen as a contraindication to UKA; however, with the metal-backed design of the Oxford, obesity is not considered a contraindication.
The current list of contraindicated conditions includes :
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Inflammatory arthropathy
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Previous high tibial osteotomy (opening or closing wedge)
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ACL deficiency
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Medial collateral ligament (MCL) contracture with inability to correct the varus deformity
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Weight-bearing cartilage wear of the lateral compartment
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Severe patellofemoral arthrosis with lateral facet disease, lateral subluxation, and trochlear grooving
Other conditions that can be successfully treated with Oxford UKA include avascular necrosis (AVN) of the medial femoral condyle, posttraumatic medial compartment arthritis, and chronic medial femoral condyle osteochondritis dissecans (OCD) ( Fig. 5 ). In order to ensure adequate femoral component fixation, careful attention must be paid to the femoral bone stock in cases of AVN and large OCD lesions. In the rare cases of medial compartment traumatic arthritis, the knee must still be ligamentously stable and the deformity correctable without MCL contracture.
Surgical technique
Most Oxford UKAs can be performed as an outpatient procedure if the patient meets medical criteria and the anesthesia is adequate. The typical anesthesia is general anesthesia coupled with a peripheral nerve block, short-acting spinal injection, and/or local pericapsular injection. The patient is typically positioned in a hanging leg holder that flexes the hip to approximately 30° and allows the operative knee to flex to 135° ( Fig. 6 ).
A paramedial incision is made from the superomedial edge of the patella to the medial border of the tibial tubercle ( Fig. 7 ). This incision can be extended proximally and distally in cases of intraoperative conversion to TKA. The anteromedial portion of the tibia needs to be exposed for tibial resection; however, the MCL must not be released. Notch and medial compartment osteophytes are important to remove and the ACL and lateral compartment should be inspected. The lateral femoral condyle should be free of any articular cartilage lesions on the weight-bearing surface. If the ACL is deficient or there is more lateral articular cartilage damage than expected, the patient can be repositioned and the incision extended for a primary TKA.
