Mobile inserts don’t improve ROM after TKA
By Sally Chapralis
A patient’s range of motion (ROM) after total knee arthroplasty (TKA) is always a critical issue. The authors of scientific paper 007 asked, “Does mobile insert contribute to knee kinematics after TKA?” Their study found that “mobile-bearing inserts have little or no contribution, at least in nonarthritic knees.”
Researchers evaluated femoral component rollback as well as tibial internal rotation, using fluoroscopy to verify the contribution of the inserts to knee kinematics after TKA. Clinical outcomes were rated and evaluated using the Knee Society Knee Score and Function Score. A single implant system, which offered both fixed- and mobile-bearing inserts, was used. In this prospective study, patients were randomly assigned to either fixed- or mobile-bearing group.
Results showed that although mobile-bearing inserts do increase tibial internal rotation in patients with rheumatoid arthritis (RA) when compared to fixed inserts, there is no significant difference for patients with osteoarthritis (OA). Furthermore, tests did not indicate improvement in femoral rollback or maximal flexion angle.
The study included 45 patients (7 males and 38 females) with a mean age of 70 years at the time of the operation. The initial diagnosis was OA for 28 patients (4 males and 24 females); the remaining 17 patients had RA (3 males, 14 females). The mean follow-up period was 17 months.
Of the 11 patients who had consecutive bilateral TKA, eight had one knee with fixed-bearing insert and the other with a mobile-bearing insert. Mobile-bearing inserts were used in 17 knees with OA and in 13 knees with RA. Fixed-bearing inserts were used in 17 knees with OA and 9 knees with RA. The study had no criteria for insert selection based on the patient’s preoperative characteristics. Approximately four weeks of postoperataive rehabilitation, including ROM exercises, was carried out.
Data included clinical outcomes, including Knee Society Scores and maximal flexion angle; and fluoroscopic results, such as femoral component rollback and tibial rotation angle. They were compared statistically by Student’s t-test regarding insert types and disease diagnoses. The authors also analyzed the femoral component rollback as well as tibial internal rotation of more than 90° flexion, for correlation with maximal flexion angle. A value less than 0.05 was regarded as significant in each analysis.
In measuring maximal flexion angle, the presenter did not see a “statistically significant difference regarding either insert types or diagnoses.” However, the Knee Society Function Score was 89.7 for patients with OA who received fixed inserts and 76.8 for patients with OA who had mobile inserts, which is a significant difference. This compares to Function Score of 84 for patients with RA who had fixed inserts and 82.5 for patients with RA who had mobile-bearing inserts. As for Knee Society Knee Score, no significant differences were recognized either regarding insert types or regarding disease diagnoses.
Femoral component rollback of greater than 90° was demonstrated in deep flexion in all subgroups, but no differences were observed between insert types or between diagnoses. The authors note that, because only a small sample was studied, a larger number of patients could produce another result. And, they added, “contribution of muscle contraction to rollback would also be an issue to be studied because we studied knees in passive flexion with non-weight-bearing.”
Tibial internal rotation revealed other differences. Patients with RA who received mobile inserts had significantly more tibial internal rotation, not only than patients with OA with mobile inserts, but also than patients with RA with fixed inserts. Patients with RA who received mobile inserts also exhibited a correlation with maximal flexion angle. The authors note, “in TKAs with more internal rotation of tibia over 90° flexion, more maximal flexion angle could be expected,” but this was only the case in patients with RA with mobile inserts.
The authors reported that while it is always difficult to reach consistent conclusions from small samples of patients, their study does recommend caution when considering use of mobile inserts in knees with vulnerable soft tissues, such as those in patients with RA. Furthermore, they add, since “no kinematical differences were noted between insert types, and Knee Society Function Scores were significantly better in TKAs with fixed insert, there could be no special reason for selection of mobile inserts, at least in TKAs for nonarthritic knees.”
The research team includes Kenrin Shi, MD, Kenji Hayashida, MD, Hideo Hashimoto, MD and Susumu Saito, MD, all of whom are from Osaka, Japan. The authors report no conflicts of interest.