Domed lateral UKA
Weston-Simons et al. (2014) reported the results of the first 265 domed lateral OUKA implanted by the two designer surgeons. These were all implanted for isolated disease of the lateral compartment and were prospectively and independently assessed. In this series, the mean age of patients at surgery was 64 years (range 32 to 90). At a mean follow-up of four years (0.5 to 8.3) the mean final OKS was 40 (SD 7.4). A total of 13 knees (4.9%) had re-operations, of which four (1.5%) were for dislocation. All dislocations occurred in the first two years. Two (0.8%) were secondary to significant trauma that resulted in ruptured ligaments, and two (0.8%) were spontaneous. In four patients (1.5%), the UKA was converted to a primary TKA. Survival at eight years, with failure defined as any revision, was 92.1% (95% CI 81.3 to 100).
There have been four independent studies of the domed lateral OUKA, which have confirmed that good results can be achieved (Table 12.1). In 2012 Streit et al. (2012) reported on the results of their first 50 domed lateral UKRs in 50 patients with a mean follow up of 3 years (minimum 2 years) and no patient lost to follow up. There were three dislocations (6.1% (95% CI 2.0 to 17.9)). Survival using revision for any reason and aseptic revision was 94% (95% CI 82 to 98) and 96% (95% CI 85 to 99) at three years, respectively. The mean Oxford knee score was 43 (SD 5.3), the mean Objective American Knee Society score was 91 (SD 13.9) and the mean Functional American Knee Society score was 90 (SD 17.5). The mean maximum flexion was 127° (range 90° to 145°).
Schelfaut et al. (2013) described a cohort of 25 patients with a minimum fol- low-up of 1 year who underwent lateral UKA with the domed Oxford. One bearing dislocation occurred 4 months post-surgery. The OKS improved in all patients from a preoperative mean of 23.3 (range 8 to 40, SD 8.4) to a postoperative mean of 42.1 (range 23 to 48, SD 6.7). General patient satisfaction at a mean follow-up of 20 months was excellent in 84 %, good in 12 % and fair in 4 %.
Also in 2013, Altuntas et al. (2013) described a total of 64 knees, in 58 patients, after a domed tibia, mobile bearing lateral UKA with a minimum 2-year follow-up. The mean follow-up period was 38 months (range 24 to 61, median 36). There were four knees that underwent further surgery for any reason. Two patients required revision of the implant (3.1%). There were no cases of bearing dislocation in this series. The mean pre-operative OKS was 24 (range 9 to 36) and the mean post-operative score was 42 (range 23 to 48 p<0.0001).
Table 12.1 Dislocation rates in four independent series of domed lateral arthroplasties.
Author |
Year |
No. of Knees |
Mean FU in months |
No. of dislocations |
Dislocation rate |
2012 |
50 |
36 |
3 |
6.2% |
|
2012 |
68 |
38 |
0 |
0% |
|
2012 |
25 |
20 |
1 |
4% |
|
2013 |
12 |
35 |
1 |
8% |
|
2014 |
265 |
48 |
4 |
1.5% |
|
Sum/average |
|
430 |
43 |
9 |
2% |
The combination of the modified surgical technique and new design with a domed tibial component appears to have reduced the early dislocation rate. It is however still higher than in the medial compartment. There is also variation in the reported dislocation rates in the published series with the surgeons doing larger numbers having a lower dislocation rate (Table 12.1). Although there are no significant difference in rates, it does suggests that the dislocation rate is influenced by surgical experience and technique.
Gulati et al. (2014) carried out a radiographic evaluation of surgical factors affecting bearing dislocation. Aligned post-operative anteroposterior knee radiographs of seven knees that had dislocated and 87 control knees were compared. All bearing dislocations occurred medially over the vertical wall of the tibial component into the intercondylar notch (Fig. 12.2). Knees that dislocated tended to be overcorrected compared with those that did not dislocate:, they were in 2° less valgus (p = 0.019) and the tibial components were positioned 2 mm more proximal (p < 0.01). Although the relative position of the centre of the femoral component and the tibial component was the same (p = 0.8), in the dislocating group the gap between the edge of the femoral component and the top of the wall in flexion was 3 mm greater (p= 0.019) suggesting that the components were internally rotated.
This study confirmed the findings of the previous study that to minimise the risk of dislocation, the knee should not be overstuffed (Robinson et al., 2002). This is best achieved by selecting the bearing thickness that just tightens the ligaments in full extension. In addition, the size of the gap between the femoral and tibial components through which the bearing dislocates should be minimised. We used to rotate the femoral component internally so as to minimise the risk of anteromedial overhang, this however increases the size of the gap. It therefore seems sensible to implant the femoral component in neutral rotation. This is best achieved by using the microplasty femoral drill guide. We have therefore modified the technique to include this. It should however be noted that this is a recent change so we do not have any outcome data and therefore cannot be certain if it helps decrease the dislocation rate. There is always a concern that a change to the technique may worsen the results.