The Oxford medial unicompartmental knee replacement (UKR; Zimmer Biomet) implant is commonly used to treat anteromedial knee arthritis and medial compartment osteonecrosis. The implant design and the indications and surgical technique for UKR have evolved over the past 40 years, with the Phase 3 implant design being introduced in 19981. Excellent long-term survival and clinical outcomes, including at 10 years postoperatively with the Phase 3 implant1,2, have been demonstrated with the Oxford UKR implant3.
While still debated, the common teaching for routine total knee replacement is to strive for neutral limb alignment with component position perpendicular to the mechanical axis. It is commonly accepted that leaving a limb in varus after total knee replacement could lead to earlier failures. In contrast, the Oxford UKR implant facilitates loading through the center of the components, with prior data suggesting that varus alignment of the limb does not compromise clinical or radiographic outcomes4.
Kennedy et al. sought to build on this earlier work4 to further establish the relationship of limb alignment with patient-reported outcomes and revision risk with the Oxford UKR implant. They reviewed the clinical outcomes and implant survival rates following UKRs with cemented mobile-bearing Phase 3 Oxford medial implants that had been performed by 2 designer surgeons. One thousand UKRs were performed, 97.7% of which were for the treatment of anteromedial osteoarthritis; 109 knees were excluded. Intraoperatively, the surgeons aimed to restore native ligament tension and, by extension, the prearthritic knee condition. Patient-reported outcome data were analyzed and compared with coronal limb alignment, measured with long-arm goniometers by independent physiotherapists. Patients were followed for an average of 10 years (range, 5 to 17 years) postoperatively.
The authors found 8%, 35%, and 57% of the limbs treated with UKRs in marked varus, mild varus, and neutral alignment, respectively. There was no association between limb alignment and 5 and 10-year postoperative Oxford Knee Scores, Tegner activity scores, or American Knee Society Scores. However, when evaluated according to the percentage of patients with good or excellent Oxford Knee Scores, increasing varus alignment was associated with improved outcomes after 10 years. Survival data showed no significant differences in 12-year revision risk between alignment groups.
In light of these findings, Kennedy et al. recommend restoring normal ligament tension, and, therefore, the patient’s prearthritic knee alignment, which is frequently varus. In their series, 42% of patients had either mild or marked varus, and the authors suggest that correcting the alignment of these limbs to neutral should be avoided. Further, the authors posit that since patients with limbs in postoperative varus appear to do as well as, if not better than, patients with limbs in neutral alignment, marked tibia vara should not be considered a contraindication for the Oxford UKR.
One of our goals as arthroplasty surgeons is to perform a procedure with precision, accuracy, and reproducibility. However, these finding appear to indicate that the target alignment for the Oxford UKR may not be a fixed value, but rather, is patient-specific. While soft-tissue tension is assessed intraoperatively, prearthritic knee alignment does not represent a precise target, as such alignment is unknown in most of our patients and, therefore, is not easily reproducible. Additionally, if the baseline alignment of a patient limb is mild or marked varus and there is a slight deviation in surgical accuracy, postoperative alignment may be extreme, potentially compromising patient-reported and survival outcomes. Nevertheless, by following the Oxford UKR designers’ surgical techniques with proper ligament tensioning, postoperative limb alignment has been shown to be comparable with that of the unaffected, contralateral limb5, suggesting that the Oxford UKR does restore native alignment.
Restoring native patient knee kinematics and alignment intuitively makes sense, and the success of the Oxford UKR is a testament to that. Since disease progression in the lateral compartment is one of the most common causes of UKR failure, varus alignment would be protective against lateral compartment disease, supporting superior outcomes in patients with residual varus. However, 10 years is only the beginning of the expected “lifespan” of the Oxford UKR implant, and future research is needed to determine whether the early success seen in this series holds up in nondesigner series and to the test of time. Meanwhile, patients with limbs in constitutional varus that remain in varus postoperatively will likely have successful outcomes at 10 years and probably longer. Yet, as always, we must remain careful with our patient selection and surgical techniques as well as be cautious about expanding the indications for this popular implant.
1. Mohammad HR, Strickland L, Hamilton TW, Murray DW. Long-term outcomes of over 8,000 medial Oxford Phase 3 unicompartmental knees-a systematic review. Acta Orthop. 2018 Feb;89(1):101-7. Epub 2017 Aug 23.
2. Alnachoukati OK, Barrington JW, Berend KR, Kolczun MC, Emerson RH, Lombardi AV Jr, Mauerhan DR. Eight hundred twenty-five medial mobile-bearing unicompartmental knee arthroplaties: the first 10-year US multi-center survival analysis. J Arthroplasty. 2018 Mar;33(3):677-83. Epub 2017 Oct 16.
3. Price AJ, Svard U. A second decade lifetable survival analysis of the Oxford unicompartmental knee arthroplasty. Clin Orthop Relat Res. 2011 Jan;469(1):174-9.
4. Gulati A, Pandit H, Jenkins C, Chau R, Dodd CA, Murray DW. The effect of leg alignment on the outcome of unicompartmental knee replacement. J Bone Joint Surg Br. 2009 Apr;91(4):469-74.
5. Mullaji AB, Shah S, Shetty GM. Mobile-bearing medial unicompartmental knee arthroplasty restores limb alignment comparable to that of the unaffected contralateral limb. Acta Orthop. 2017 Feb;88(1):70-4. Epub 2016 Oct 31.