Finite-Element Analysis of Stress on the Proximal Tibia After Unicompartmental Knee Arthroplasty.

BACKGROUND: Because the indications for unicompartmental knee arthroplasty (UKA) are limited, few patients have undergone the procedure. Therefore, it is difficult to decide the acceptable range of variation in the details of UKA on the basis of the available clinical data. The objective of this study was to identify factors that affect the distribution of stress on the proximal tibia after UKA. METHODS: Two-dimensional finite-element analysis of the proximal tibia was used to assess four factors: 1) two types of implants-all ultra-high-molecular-weight polyethylene (UHMWPE) and metal-backed implants, 2) postoperative alignment, 3) coverage of tibial bone, 4) level of the tibial osteotomy. RESULTS: In cases of varus alignment, high stress values and large areas of deformation were observed on and beneath the implant. In cases of valgus alignment, stress was concentrated at the lateral portion of tibial tray. In comparison with the standard model, stress concentration was greater at the medial edge of the medial condyle in a narrow-coverage model. Stress distribution for the low-osteotomy-level model did not differ markedly differ from that for the standard model. Stress distribution was better for metal-backed implants than for UHMWPE implants. CONCLUSIONS: Proper postoperative alignment must be achieved in UKA. The osteotomy level should be set at the cancellous bone close to the joint line, and preservation of bone stock should be maximized.

Stress fractures of the femoral neck in child with open capital femoral epiphysis.

Stress fracture of the femoral neck has been considered a typical injury of two age groups: active youths and elderly people with osteoporosis. Its occurrence in children with an open capital femoral epiphysis is rare, and only 11 cases have been reported. Here we report two cases, in a 12-year-old boy and a 5-year-old girl (among the youngest patients on record). This fracture can be classified as a compression fracture or a tension fracture. Although tension fractures are not uncommon in military recruits and young athletes, all 13 cases (including our two cases) of femoral neck stress fracture associated with an open capital epiphysis were compression fractures that were treated conservatively.