The Exeter cemented polished tapered stem design was introduced into clinical practice in the early 1970's. [i] Design and cement visco-elastic properties define clinical results [ii]; a recent study by Carrington et al. reported the Exeter stem has 100% survivorship at 7 years. [iii] Exeter stems with offsets 37.5–56 mm have length 150 mm (shoulder to tip). Shorter stems, lengths 95–125 mm, exist in offsets 30–35.5 mm. The Australian National Joint Replacement Registry recently published that at 7 years the shorter stems are performing as well as longer stems on the registry [iv]. Clinical observation indicates in some cases of shorter, narrower femora that fully seating a 150 mm stem's rasp in the canal can be difficult, which may affect procedural efficiency. This study investigates the comparative risk of rasp distal contact for the Exeter 150 mm stem or a 125 mm stem. Rasps for 37.5, 44, 50 mm offset, No.1, 150 mm length stems (Exeter, Stryker Orthopaedics, Mahwah NJ) were compared with shortened length models using SOMA™ (Stryker Orthopaedics Modeling and Analytics technology). 637 patients' CT scanned femora were filtered for appropriate offset and size by measuring femoral-head to femoral-axis distance and midsection cancellous bone width (AP view). These femora were analyzed for distal contact (rasp to cortices) for 150 mm and 125 mm models (Figure 1). The widths of the rasp's distal tip and the cancellous bone boundary were compared to assess contact for each femur in the AP and ML views; the rasp was aligned along an ideal axis and flexed in order to pass through the femoral neck (ML view only).Introduction
Materials and Methods
Recreating the natural head center of the hip joint during hip arthroplasty is important for restoring biomechanics in order to minimize leg length discrepancies, improve soft-tissue tension, and mitigate impingement [1,2]. New tools have been developed that allow anatomical measurements and analysis of three-dimensional digital femura geometry based on CT scans [3]. The purpose of this study is to analyze the head center location of various fit-and-fill hip stem designs in relation to the natural bone head center location using a novel technique. 556 computer tomography (CT) images (SOMA™) of left femora were used in this study. The acetate templates of five fit-and-fill stem designs (Design 1: Secur-Fit Advanced, Stryker; Design 2: Secur-Fit Max, Stryker; Design 3: Summit, Depuy; Design 4: Synergy, Smith & Nephew; Design 5: Zimmer, VerSys Epoch FullCoat) were compared to each other to correlate stem sizes between different systems. The appropriate stem body size for each of the CT bones was established based on the medial offset of the bone 20 mm above the lesser trochanter (MO+20) and the stem medial offset at the medial resection point. Utilizing the commercially available offset heads for each design, the bone head centers and the stem head centers were plotted, aligning the central axis of the bone/stem as well as the MO+20 of the bone with the stem medial resection point. The percent of bone head centers within 1,2,3,4,&5 mm of a stem/head offset data point was calculated for all designs. Additionally, the distance from the bone head center to the closest stem/head offset data point and the average head offset used were calculated.INTRODUCTION:
METHODS:
Immediate post-operative stability of a cementless hip design is one of the key factors for osseointegration and therefore long-term success [1]. This study compared the initial stability of a novel, shortened, hip stem to a predicate standard tapered wedge stem design with good, long-term, clinical history. The novel stem is a shortened, flat tapered wedge stem design with a shape that was based on a bone morphology study of 556 CT scans to better fit a wide array of bone types [2]. Test methods were based on a previous study [3]. Five stems of the standard tapered wedge design (Accolade, Stryker Orthopaedics, Mahwah, NJ) and the novel stem (Accolade II, Stryker Orthopaedics, Mahwah, NJ) were implanted into a homogenous set of 10 synthetic femora (Figure 1) utilizing large left fourth generation composite femurs (Sawbones, Pacific Labs, Seattle, WA). The six degrees-of-freedom (6 DoF) motions of the implanted stems were recorded under short-cycle stair-climbing loads. Minimum head load was 0.15 kN and the maximum load varied between 3x Body Weights (BW) and 6 BW. Loading began with 100-cycles of “normal” 3 BW and was stepped up to 4 BW, 5 BW & 6 BW for 50-cycles each. Prior to each load increase, 50 cycles of 3 BW loading was applied. This strategy allowed a repeatable measure of cyclic stability after each higher load was applied. The 6 DoF micromotion data, acquired during the repeated 3 BW loading segments, were reduced to four outcome measures: two stem migrations (retroversion and subsidence at minimum load) and two cyclic motions (cyclic retroversion and cyclic subsidence). Data were analyzed using repeated measures ANOVA with a single between-subjects factor (stem type) and repeated measures defined by load-step (3 BW, 4 BW, 5 BW 6 BW).INTRODUCTION
METHODS
