Abstract
Introduction
Short stems have been developed for some years for preservation of femoral bone stock and achieve physiological proximal loading. Shortening stem length is a merit for bone stock preservation. However, it might lead to reduction of primary stability. We investigated relationship between stem length and primary stability by patient specific finite element analysis (FEA).
Materials and Methods
Thirty-one hips in 31 patients were performed total hip arthroplasty with standard length tapered wedge-shaped (TW) cementless stem (CTi-II: Corin, Cirencester, UK). There were 6 males and 25 females. The average age at operation was 69 years old. The average body mass index was 23.9 kg/m2. Primary diagnoses were secondary osteoarthritis due to developmental dysplasia of the hip in 29 hips. Femoral canal shapes were normal in 21, stovepipe in 6 and champagne-flute in 4 hips. Bone qualities were type A in 6, B in 19 and C in 6 hips.
The patients underwent computed tomography (CT) preoperatively and postoperatively. We constructed preoperative three dimensional (3D) femur surface models from preoperative CT data with individual bone mineral density (BMD) mapping. The postoperative 3D femur and rough stem surface models were obtained from postoperative CT data. The coordinates of the postoperative femur were transformed to fit the preoperative femur model. A precise stem model constructed using computer-assisted design data was matched to the transformed rough stem model using the iterative closest point algorithm. We obtained a patient-specific model with the proximal bone geometry, allocation of BMD and stem alignment. We estimated the average of axial and rotational micromotion (MM) at stem-bone interface and the ratio of area (MM � 40 micrometers) on the porous surface in order to analyze primary stability of TW stem with several lengths (standard (100 %), 75 %, 50 %, 40 % and 30 % length).
Results
The average MM in standard length stem was 14.3 micrometers and the ratio of area with MM â�¦ 40 micrometers was 97.9 %. The average of axial and rotational MM in shorter length (75 %) stem were respectively 9.7, 8.3 micrometers. There were no differences in the average of axial and rotational MM between standard and shorter (75 %) length stems. MM at the porous surface was increased as the stem length grew shorter. The ratio of area with MM â�¦ 40 micrometers on the porous surface were reduced by 50 to 80 % in −40 % or less length stem, comparing with the standard length stem.
Discussion and Conclusion
The present FEA on the stem length and MM demonstrated that primary stability in 40 % or less short length TW stem was extensively reduced, which might lead to failure of bone ingrowth on the porous surface and early loosening. Shortening of stem length less than 50 % is a risk for reduced primary stability in TW stem.
