Abstract
Background
The purpose of this study was to analyze the effect of femorotibial alignment (FTA), femoral and tibial component alignment, correction of malalignment, and thickness of tibial osteotomy on implant loosening following total knee replacement.
Methods
We retrospectively reviewed 107 knees in 65 patients with a minimum of six months of follow-up. The 107 knees were operated by two surgeons using BS4+ (Bisurface 4 plus) implant (Japan Medical Materials, Japan); the femoral component was cemented, and the tibial component was either cemented or not cemented by using four screws. All the replacements were performed under same operative procedure with medial para-patellar approach and measured bone technique. The knees were classified into two groups (: I and U) on the basis of postoperative radiological findings that indicate the loosening of tibial components. First, there were not any apparent loosening symptom like radio-lucent lines nor sinking; group-I (intact, n=75). Second, there were some radio-lucent lines around tibial component; subgroup-R (radio-lucent lines, n=25), or some subsidence of component over 2 mm; subgroup-S (subsidence, n=7), and the latter two subgroups were put into group-U (unstable, n=32) all together. We measured preoperative and postoperative alignment (overall FTA, correction of malalignment, and alignment of the tibial and the femoral component in the coronal plane). Furthermore, each thickness of tibial osteotomy was measured with use of preoperative and postoperative radiographs of the knee. These parameters including patient's BMI were compared between two major groups statistically to evaluate the factor influencing the stability of tibial components. Moreover, the thickness of tibial osteotomy were compared between two subgroups.
Results
Radio-lucent lines were seen in any fixation (cement: 14, cementless: 11), but subsidence were found in 7 cementless screwed fixation. Average alignment of femorotibia or component were satisfactory (judged by the Knee Society Roentgenographic Evaluation), and were not different between two groups.
We found that the change of FTA (I: 9.5 versus U: 12.9 degrees) and the outlier of FTA (I: 2.4 versus U: 3.1 degrees) were greater in group-U than those in group-I (p=0.002, p=0.023). In the thickness of tibial osteotomy, no significant differences were seen between two major groups, but the osteotomy were thicker in subgroup-S (12.0 mm) than in subgroup-R (9.6 mm, p = 0.03). Another parameters including BMI were not different between two groups, respectively.
Conclusions
Attaining neutrality is important in stabilizing tibial component of total knee replacement. However, the larger the correction of malalignment, the more unstable the fixation of component tend to become. High-flexion femoral component designs including BS4+ require the removal of 2 to 3 mm more bone from the component-bone interface than with standard implants, generally. Nevertheless, thicker osteotomy of tibia may be a risk factor of subsidence of tibial component. We should take into account cement fixation in such cases like severe malalignment or thicker osteotomy in total knee replacement.
