The average longevity of a HTO has been estimated to be about six to seven years, with conversion to total knee arthroplasty (TKA) occurring in about 20% of patients after 5 years, 40% after 10 years, and 60% after 15 years [6]. Patella baja, change of tibial slope and valgus alignment after HTO can all cause technical difficulties during TKA, particularly in relation to eversion of patella, exposure of the lateral compartment, and placing the tibial component in the correct position both for rotation and slope [7].
In the present study, patella descent occurred in all knees early after HTO, with the BP index decrease from 0.78 to 0.55. The patellar height was maintained until the latest follow-up, with BP index of 0.59. The BP indexes of three knees (0.38, 0.44 & 0.49) were outside normal range (0.54-1.06) and were defined as patella baja. The patella tendon length was insignificantly increased early after operation. It was maintained until the latest follow-up. This pointed to a structural cause of patellar descent, rather than a biological cause due to scarring. Moreover, the size of ABG was found to be significantly but inversely related to the BP index change. The larger the ABG, the larger was the patella descent, the smaller was the latest BP index, and the more negative would be the BP change. Rigid fixation and early mobilization in the present cases could minimize the patellar tendon scarring. This echoed the idea of Brouwer et al that elevation of the tibial plateau after open-wedge osteotomy was one of the cause of patella baja [7]. Alteration of tibial slope was reported to affect the patellar height [6, 7]. It could not be concluded in the present study (r = 0.204, p = 0.599). Moreover, the patellar height was not found to be correlated to the final functional and knee score.
Tibial slope in the current study was reported by the method of Moore and Harvey [11] because lateral radiograph of the whole tibia was not available to draw the mechanical tibial axis. Moreover, it was reproducible in the lateral knee radiograph by using the anterior tibial cortex and tibal plateau surface as landmarks. The posterior tibial slope was not significantly altered after HTO. It was maintained after complete union of bone graft in the latest follow-up. A few factors contributed to the success. Osteotomy cut was confirmed by x-ray to be parallel to the tibial plateau surface. The knee was kept in full extension when measurement of bone graft sizes was done, and also during the fixation of osteotomy by plate. Lateral x-ray of the contralateral knee was obtained to compare the tibial slope. If there was pre-existing knee flexion contracture and the osteotomy was used to correct that deformity, the posterior tibial slope would be diminished.
Over-correction of coronal valgus alignment was important to obtain satisfactory result after HTO [3, 13]. Hernigou et al reported that the best results were obtained in 20 cases of open-wedge HTO that had mechanical hip-knee-ankle angle of 183° to 186° [3]. There was no pain and no progression of the arthrosis in either the medial or the lateral tibiofemoral compartment. Aglietti et al reported the results of 91 closing-wedge HTO [13]. An anatomical valgus alignment at consolidation between 8° and 15° was significantly correlated with the best result. In the current study, the former recommendation was followed because it was more stringent and open-wedge HTO was performed. In this way, the Knee Society knee score was also modified because the target range in HTO was different from that in TKA. The mechanical coronal alignment was significantly corrected from varus 10.5° to valgus 4.4°. At consolidation of bone graft, it was maintained at valgus 3.9°. The varus recurrence was minimal and not significant. Six out of nine knees could achieve the target alignment. One knee had varus 1° and the patient had minimal knee pain and his knee score was 86 at 24 month follow-up. Two knees had valgus 7° and the patients had no knee pain. One of their knee scores was 98 at 36 month follow-up and the other was 87 at 27 month follow-up. In order to achieve the target alignment, a few points were important, including accurate preoperative planning, careful prevention of lateral tibial translation on opening up the osteotomy and measurement of bone graft size, harvesting graft larger than measured size to allow down-trimming and rigid internal fixation.
The normal knee joint has varus 3° obliquity. The preoperative value of the present cases was varus 5.5° (2°-10°) (p = 0.023). It indicated significant medial compartment bone loss. It was changed to valgus 7.7°(4°-11°) at bone graft consolidation. One knee had valgus 11°. Hernigou et al found that their maximum postoperative obliquity was 10° [3]. After follow-up of eleven years, no medial subluxation of tibia was noted.
The knee range of motion was not altered significantly after HTO, while the function and knee scores were significantly improved. Only one patient had deterioration of function score after HTO. His function score was 60 before his left HTO, which ameliorated his left knee pain. He had exacerbation of right knee pain and the function score was 55 before his right HTO at about eight months later. In the most recent follow-up at 22 months after first HTO, the function score decreased to 45. The latest HKAs were 183° and 185° and the knee scores changed from 47 and 45 to 75 and 70 for right and left sides respectively. He had exacerbation of bilateral patellofemoral joint pain related to increasing osteoarthritic change, which affected the knee score. In addition, he had bilateral calves pain related to spinal problem, which affected his functional status. This last case exemplified the importance of patient selection in achieving good postoperative result. Even with ideal alignment, postoperative knee pain exacerbation and functional deterioration could happen.