Although total hip arthroplasty is a very successful operation, complications such as: dislocation, aseptic loosening, and periprosthetic fracture do occur. These aspects have been studied in large populations for traditional stem designs, but not for more recent short stems. The design rationale of short stems is to preserve bone stock, without compromising stability. However, due to their smaller bone contact area, high peak stresses and areas of stress shielding could appear in the proximal femur, especially in the presence of atypical bone geometries. In order to evaluate this aspect, we quantified the stress distribution in atypical proximal femurs implanted with a commercially available calcar guided short stem. Geometrical shape variations in neck-shaft angle (NSA), neck-length (NL) and anteversion (AV), were determined three-dimensionally in the Mimics Innovation Suite (Materialise N.V., Leuven, Belgium) from a CT dataset of 96 segmented femurs. For each shape variation, the femurs that had the two lowest, two average and two highest values were included (18 femurs). Using scripting functionality in Mimics, CAD design files of the calcar guided Optimys short stem (Mathys, Bettlach, Switzerland) were automatically sized and aligned to restore the anatomical hip rotation center. Stem size and position were manually corrected by an orthopedic surgeon before finite element (FE) models were constructed using a non-manifold assembly approach (Figure 1). Material properties were estimated from the CT dataset and loads representing walking and stair climbing were applied [1]. Stress-shielding was evaluated by the change in average strain energy density pre- and post-operatively in three different regions (calcar, midstem, tip) each being subdivided in four quarters (medial, lateral, anterior, posterior) (Figure 2).Introduction
Methods
Natural population variation in femoral morphology results in a large range of offsets, anteversion angles and lengths. During total hip arthroplasty, accurate restoration of hip biomechanics is essential to achieve good functional results. One option is to restore the anatomic hip rotation center. Alternatively, medializing the rotation center and compensating by increasing the femoral offset, reduces acetabular contact forces and increases the abductor lever arm. We investigated the ability of two cemented stem systems to restore hip biomechanics in an anatomic and medialized way. We compared an undersized “Exeter-type” of stem with three offset options and 18 sizes (CPT, Zimmer), to a line-to-line “Kerboul-type” of stem with proportional offset and 12 sizes (Centris, Mathys). Thirty CT scans of whole femora were segmented and the hip rotation center, proximal femoral axis and femoral length were determined with Mimics and 3-matic (Materialise). Using scripting functionality in the software, CAD design files of both stems were automatically sized and aligned along the proximal femoral axis to restore an anatomical and a 5 mm medialized hip rotation center. Stem size and position could be fine-tuned manually. The maximum distances between the prosthetic (PRC), the anatomic (ARC) and the medialized hip rotation center (MRC) were calculated (Fig. 1). Variations in femoral offset (ΔFO), anteroposterior (ΔAP) and proximodistal distance (ΔPD) were analyzed. Finally, the number of cases where the hip rotation center could be restored within 5 mm was reported.Introduction
Methods
Accurate detection of migration of hip arthroplasty stems without the burden of bone markers and stereo-radiographic equipment is of interest. This would facilitate the study of stem migration in an experimental setting, but more importantly, it would allow assessing stem loosening in patients with a painful hip outside a study protocol. We developed and validated a marker-free automated CT-based spatial analysis method (CTSA) to quantify stem-bone migration in successive CT scan acquisitions. First, we segmented the bone and stem within both three-dimensional images, then we pairwise registered those elements (Fig. 1). By comparing the rigid transformations of stem and bone, we calculated the migration of the stem with reference to the bone and transferred the three translation and three rotation parameters to an anatomic coordinate system. Based on the rigid transformation, we also calculated the point of the stem that presented the maximal migration (PMM). Accuracy was assessed in a stem-bone model (Fig. 2) by imposing 39 predefined stem rotations and translations, and by comparing those with values calculated with the CTSA tool. In all cases, differences were below 0.20 mm for translations and 0.19° for rotations (95% tolerance interval (95% TI) below 0.22 mm and 0.20°, largest standard deviation of the signed error (SDSE) 0.081 mm and 0.057°). Precision was defined as stem migration calculated in eight clinical relevant zero-migration scenarios. In all cases, precision was below 0.05 mm and 0.08° (95% TI below 0.06 mm and 0.08°, largest SDSE 0.012 mm and 0.020°). The largest displacement of the PMM on the stem was 0.169mm. The precision estimated in five patients was very dependent on the CT scan resolution and was below 0.48 mm and 0.37° (95% TI below 0.59 mm and 0.61°, largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.972 mm). In optimized conditions, the precision in patients improved largely and was below 0.040 mm and 0.111° (largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.156 mm). Our marker-free automated CT-based spatial analysis can detect hip stem migration with an accuracy and precision comparable to that of radiostereometric analysis (RSA), but without the burden of bone markers and the cost of stereo-radiographic equipment. As such, we believe our tool could make accurate measurement of stem migration available to departments without access to RSA and boost this type of research. Moreover, as CTSA does not rely on bone makers, it is applicable to all-comers with a painful hip arthroplasty. Indeed, in those patients with a reference CT scan after hip replacement, a new CT scan could demonstrate stem migration. If no initial CT scan is available, a reference scan could be taken during a first visit and repeated later. Additionally, a “stress test” of the hip could be performed. During such test, comparing CT images acquired during forced maximal intern and external rotation could demonstrate stem loosening.
Thermal necrosis of the femoral head, due to heat generation during cement polymerization, is a concern in hip resurfacing. Bone necrosis could cause fractures and/or implant loosening. Some authors1 found an inverse relationship between the size of the femoral component and the risk of revision after hip resurfacing. We postulate that smaller implants contain proportionally more cement than larger ones and that this could explain the effect of implant size on revision rate. As such, we investigated the relation between implant size and both, the average cement mantle thickness and the cement-filling index (fraction of cement volume and total volume within the implant). Nineteen human femoral heads, collected during total hip arthroplasty, were machined for hip resurfacing with original ReCap (Biomet) instruments. The head sizes were chosen so we could implant two resurfacing heads for each even size between 40 and 56 mm, and one for size 58 mm. Each reamed head was provided with a number of anchoring holes proportional to the head size and was kept at 37°C. After pressure-lavage with water at 20°C, polymeric replicas of the original Recap implants were cemented according to a strict protocol. The exact amount of Refobacin Bone Cement LV (Biomet) needed to fill half the volume of the implant was pored into the resurfacing head and 2.5 minutes after starting cement mixing, the implant was manually impacted on the reamed femoral head. Specimens were scanned with computer tomography from the distal border of the resurfacing head to the top of the dome and CT-images were analyzed with an adapted version of validated segmentation software2. Based on gray values we identified four different elements: the polymeric stem and the outer shell of the implant, the cement-free cancellous bone and the cement mantle. Both, the average cement mantle thickness and the cement-filling index were calculated as described previously3.INTRODUCTION
MATERIALS AND METHODS
Hip resurfacing arthroplasty has gained popularity as an alternative for total hip arthroplasty. Usually, cemented fixation is used for the femoral component. However, each type of resurfacing design has its own recommended cementing technique. In a recent investigation the effect of various cementing techniques on cement mantle properties was studied. This study showed distinct differences in cement mantle volume, filling index and morphology. In this study, we investigated the effect of these cement mantle variations on the heat generation during polymerization, and its consequences in terms of thermal bone necrosis. Two FEA models of resurfacing reconstructions were created based on CT-data of Thermal analyses were performed of the polymerization process, simulating three different types of bone cement: Simplex P (Stryker), CMW3 (DePuy J&J) and Osteobond (Zimmer), with distinct differences in polymerization characteristics. The polymerization kinematics were based on data reported previously. During the polymerization simulations the cement and bone temperature were monitored. Based on the local temperature and time of exposure, the occurrence of thermal bone necrosis was predicted. The total volume of necrotic bone was calculated for each case.Introduction
Materials and methods
