Unicompartmental knee replacement (UKR) has good outcomes for the treatment of compartmental osteoarthritis of the knee. Mechanical alignment overcorrection is associated with early failure of the femoral and tibial components. Preoperative mechanical alignment is the most important predictor of postoperative alignment. However, most studies do not take into consideration the magnitude of preoperative deformity when reporting on mechanical alignment outcomes after UKR. We aimed to determine the magnitude of postoperative mechanical alignment achieved based on the magnitude of preoperative alignment; and to compare the number of cases of overcorrection into valgus to historical data. This was a radiographic review of patients who underwent robotic medial UKR by a single surgeon between 2007 and 2011. Two examiners measured pre- and postoperative mechanical alignment for all patients on long-leg radiographs. Patients were classified into three groups of preoperative mechanical alignment: mild varus (0–5®); moderate varus (5–10®); and severe varus (>10®). Patients with valgus alignment (<0®) were excluded. Linear regression was used to estimate the magnitude of postoperative alignment for each group, adjusting for age, BMI, gender, side, implant type, and polyethylene thickness. 89 patients were included. Mean preoperative alignment was 7.3® varus (95% CI = 6.6®–8®; range, 0.1–15® varus). Mean postoperative alignment was 2.8® varus (95% CI = 1.9®–3.8®; range, 1.4® valgus–9.7® varus). There was a significant difference in postoperative mechanical alignment between the three groups (Table 1) (P<0.05). Four overcorrections (4.5%) were detected, all under 1.5® valgus. This percentage of overcorrection was significantly better than previous conventional UKR reports (mean = 12.6%; P = 0.04). The magnitude of postoperative alignment in medial UKR depends on the severity of the preoperative deformity. Reports on radiographic outcomes of UKR should be stratified by the magnitude of preoperative alignment. The risk of overcorrection is reduced when using robotic assistance compared to using the conventional manual technique.
The anterior femoral cortical line (AFCL) is an anatomical landmark which has been used by the senior author for 20 years to assess femoral rotation in over 4000 TKR's. The AFCL describes the alignment of the anterior cortex of the distal femur proximal to the trochlear articular cartilage. The AFCL was compared with the surgical epicondylar (SEA), anteroposterior (Whiteside's line) and posterior condylar (PC) axes using 50 dry-bone cadaveric femora, 16 wet cadaveric specimens, 50 axial MRI's and 58 TKR patients intra-operatively. In the dry-bone/cadaveric femora (measuring relative to the SEA the AFCL and Whiteside's AP axis were 1° externally rotated and the PC axis was 1° internally rotated. By MRI (relative to the SEA) the AFCL was 8° internally rotated, Whiteside's was 2° externally rotated and the PC axis was 3° internally rotated. In the clinical study (measuring relative to a perpendicular to Whiteside's line alone) the AFCL was 4° degrees internally rotated, which equates to 2-3° of internal rotation relative to the SEA. The AFCL is another axis, completing the ‘compass points’ around the knee. It may prove particularly useful when one or all of the other reference axes are disturbed such as in revision TKR, lateral condylar hypoplasia or where there has been previous epicondylar trauma. We suggest building in 5° external rotation with respect to the anterior femoral cortical line for femoral component rotation.
