Computer-based motion simulation of total hip prostheses with ceramic-on-ceramic wear couple. Analysis of implant design andorientation as influence parameters

OBJECTIVE: In THA, ceramic-on-ceramic wear couples are increasingly used. A restricted range of motion (ROM) due to unfavourable implant design or positioning may cause impingement or dislocation, which can result in failure of ceramic inserts. METHODS: By means of a 3-D CAD program different hip joint movements were simulated and the effects of ceramic hip implant design and position on the range of motion were analysed. RESULTS: To offer sufficient ROM and to minimise risk of impingement and dislocation, inclination angle of the acetabular cup should be 45 degrees, cup anteversion 15 degrees and stem antetorsion 0 degrees to 10 degrees. In regard to implant design, acetabular cups with slightly- recessed ceramic inserts should be used. Prosthetic systems with an elevated liner or with a mushroom-shaped femoral head are associated with limited ROM and increased risk of mechanical failure. The ratio of head to neck diameter should never be less than 2 : 1. Larger heads not only increase ROM, but also the stability of the prosthesis against dislocation. Thereby, the wear rate of ceramic-on-ceramic couples is not increased, in contrast to polyethylene. CONCLUSION: Considering certain criteria for ceramic hip implants regarding implant positioning, design and handling, ceramic-on-ceramic couples can be used with low risk of revision surgery and they can also reduce the prosthesis loosening associated with wear in young and active patients.     

The effect of acetabular cup orientations on limiting hip rotation.

The orientation of the acetabular cup and position of lip augmentation (if present) may improve postoperative total hip replacement stability by decreasing dislocation caused by hip prosthesis impingement during rotation. To determine how these cup parameters affect dislocation, the range and amount of rotation for two standard femoral components was determined in a Sawbones hemipelvis model. The parameters that allowed for maximal range of rotation were a cup angle of inclination between 35 degrees and 45 degrees and cup anteversion between 0 degrees and 10 degrees. Cup anteversion angles greater than 20 degrees and cup angles of inclination greater than 45 degrees significantly limited internal and external rotation, particularly for hip flexion greater than 60 degrees. The position of the cup lip augmentation did not affect the amount of rotation except when the hip was flexed and the lip superiorly oriented. Although there are currently devices to facilitate a particular cup position, hip stem type and orientation are other important factors affecting range of rotation that must be considered.     

Influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations in total hip arthroplasty

Objective： To study the influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations, which meets the criteria for desired range of motion （ROM） for activities of daily living in total hip arthroplasty （THA）.
Methods： A three-dimensional generic, parametric and kinematic simulation module of THA was developed to analyze the cup safe-zone and the optimum combination of cup and neck antetorsion. A ROM of flexion ≥ 120°, internal rotation ≥ 45° at 90° flexion, extension ≥ 30° and external rotation ≥ 40° was defined as the criteria for desired ROM for activities of daily living. The cup safe-zone was defined as the area that fulfills all the criteria of desired ROM before the neck impinged on the liner of the cup. For a fixed stemneck （CCD）-angle of 130°, theoretical safe-zones fulfilling the desired ROM were investigated at different general headneck ratios （GR=2, 2.17, 2.37, 2.61 and 2.92） and femoral anteversions （FA=0°, 10°, 20° and 30°）.
Results： Large GRs greatly increased the size of safezones and when the CCD-angle was 130°, a GR〉2.37 could further increase the size of safe-zones. There was a complexinterplay between the orientation angles of the femoral and acetabular components. When the CCD-angle was 130°, the optimum relationship between operative acetabular anteversion （OA） and femoral antetorsion （FA） could be estimated by the formula： OA=-0.80×FA＋47.06, and the minimum allowable operative acetabular inclination （OImin） would be more than 2 10.5 ×GR^-2255.
Conclusions： Large GRs greatly increase the size of safe-zones and it is recommended that the GR be more than 2.37 so as to extend the acceptable range of error that surgeons cannot avoid completely during operation. As to the optimum operative acetabular inclination （OI）, surgeons need to make a decision combining with other factors, including stress distribution, soft tissue and cup wear conditions, as well as patients＇ individual situations and demands. The data obtained from this study and the module of THA can be used to assist surgeons to choose and implant appropriate implants.     

The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios

BACKGROUND: Prosthetic impingement due to poor positioning can limit the range of motion of the hip after total hip arthroplasty. In this study, a computer model was used to determine the effects of the positions of the acetabular and femoral components and of varying head-neck ratios on impingement and range of motion. METHODS: A three-dimensional generic hip prosthesis with a hemispherical cup, a neck diameter of 12.25 millimeters, and a head size ranging from twenty-two to thirty-two millimeters was simulated on a computer. The maximum range of motion of the hip was measured, before the neck impinged on the liner of the cup, for acetabular abduction angles ranging from 35 to 55 degrees and acetabular and femoral anteversion ranging from 0 to 30 degrees. Stability of the hip was estimated as the maximum possible flexion coupled with 10 degrees of adduction and 10 degrees of internal rotation and also as the maximum possible extension coupled with 10 degrees of external rotation. The effects of prosthetic orientation on activities of daily living were analyzed as well. RESULTS: Acetabular abduction angles of less than 45 degrees decreased flexion and abduction of the hip, whereas higher angles decreased adduction and rotation. Femoral and acetabular anteversion increased flexion but decreased extension. Acetabular abduction angles of between 45 and 55 degrees permitted a better overall range of motion and stability when combined with appropriate acetabular and femoral anteversion. Lower head-neck ratios decreased the range of motion that was possible without prosthetic impingement. The addition of a modular sleeve that increased the diameter of the femoral neck by two millimeters decreased the range of motion by 1.5 to 8.5 degrees, depending on the direction of motion that was studied. CONCLUSIONS: There is a complex interplay between the angles of orientation of the femoral and acetabular components. Acetabular abduction angles between 45 and 55 degrees, when combined with appropriate acetabular and femoral anteversion, resulted in a maximum overall range of motion and stability with respect to prosthetic impingement. CLINICAL RELEVANCE: During total hip arthroplasty, acetabular abduction is often constrained by available bone coverage, while femoral anteversion may be dictated by the geometry of the femoral shaft. For each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that allows the potential for a maximum range of motion without prosthetic impingement after total hip arthroplasty. These data can be used intraoperatively to determine optimum position.     

Robotic guidance in total hip arthroplasty: the shape of things to come

Surgeons want to perform a perfect total hip arthroplasty (THA) with every operation. Human performance has limitations, especially when performing a mechanical operation in a biological environment. Recent suggested changes to improve outcomes have been large femoral heads and anterior incisions, but unfortunately, neither has resulted in any scientific data that change has been effected. The scientific data tell us that poor component positions and impingement are the source of increasing mechanical complications. Therefore, attempts have been made to improve the surgeon's performance by precise quantitative knowledge in the operating room. Robotic-guided navigation provides numerical data for cup inclination plus anteversion and center of rotation; femoral leg length and offset; and combined anteversion of the cup and stem. The acetabular bone preparation is done with a reamer connected to a robotic arm, which prevents human error by the surgeon of reaming off line or too deep. This technology provides predictable and reproducible results.     

Metal-on-metal hip resurfacing: the effect of cup position and component size on range of motion to impingement

The aim of this dry bone study was to determine the range of hip motion to impingement for different hip resurfacing cup positions and component sizes. The maximum angles of hip flexion, extension, abduction, and adduction were calculated from 3-dimensional coordinates for: 1. Cup inclination of 30 degrees , 40 degrees , 50 degrees , 60 degrees , and 70 degrees with fixed anteversion; 2. Cup anteversion of 0 degrees , 10 degrees , 25 degrees , 35 degrees , and 45 degrees with fixed inclination; and 3. 3 different component sizes on the same size dry bones. An acetabular component inclination of 50 degrees and an anteversion of 25 degrees allowed the most physiologic range of hip motion. A larger-diameter femoral component relative to the native femoral neck diameter resulted in a greater range of hip motion to impingement.     

A hybrid CT-free navigation system for total hip arthroplasty.

OBJECTIVE: To design and evaluate a novel CT-free image-guided surgical navigation system for assisting placement of both acetabular and femoral components in total hip arthroplasty (THA). MATERIALS AND METHODS: The methodology in this paper is conceptually based on our previous work on CT-free cup placement. For femoral component placement, two patient-specific reference coordinate systems are first defined: One for the pelvis, based on the so-called anterior pelvic plane (APP) concept, and one for the femur, using the center of the femoral head, the posterior condylar tangential line, and the medullary canal axis of the proximal femur. A hybrid method is used for the associated landmark acquisition, which involves percutaneous point-based digitization and bi-planar landmark reconstruction using multiple registered fluoroscopy images. The following clinical parameters are computed in real time: cup inclination and anteversion, antetorsion and varus/valgus of the stem, lateralization, and change in leg length for complete THA. In addition, instrument actions such as reaming, impaction, and rasping are visualized for the surgeon by superimposing virtual instrument representations onto the fluoroscopic images. RESULTS: A laboratory study of computer-assisted measurement of antetorsion and varus/valgus, change in leg length, and lateralization for femoral stem placement demonstrated the high precision of the proposed navigation system. Compared with CT-based measurement, mean deviations of 1.0 degrees, 0.6 degrees, 0.7 mm, and 1.7 mm were found for antetorsion, varus/valgus, change in leg length, and lateralization, respectively, with standard deviations of 0.5 degrees, 0.5 degrees, 0.6 mm, and 0.7 mm, respectively. A pilot clinical evaluation showed that THA could benefit from this newly developed CT-free hybrid system. CONCLUSIONS: The proposed CT-free hybrid system promises to increase the accuracy and reliability of THA surgery.     

A New Technique for Radiographic Measurement of Acetabular Cup Orientation

Accurate radiographic measurement of acetabular cup orientation is required in order to assess susceptibility to impingement, dislocation, and edge loading wear. In this study, the accuracy and precision of a new radiographic cup orientation measurement system were assessed and compared to those of two commercially available systems. Two types of resurfacing hip prostheses and an uncemented prosthesis were assessed. Radiographic images of each prosthesis were created with the cup set at different, known angles of version and inclination in a measurement jig. The new system was the most accurate and precise and could repeatedly measure version and inclination to within a fraction of a degree. In addition it has a facility to distinguish cup retroversion from anteversion on anteroposterior radiographs.     

Gender differences in 3D morphology and bony impingement of human hips

For the proper diagnosis or treatment of hip joint disorders caused by anatomical abnormalities, the normal hip joint morphology must be studied to understand its influence on the maximum range of motion (ROM) until bony impingement by focusing on gender differences. Acetabular and femoral morphologies were analyzed from 3D CT images of 106 normal hip joints from elderly men (n = 36 joints) and women (n = 70 joints), and measurements of ROM until bony impingement were made in four directions (flexion, extension, and external and internal rotation at 90° flexion) using surface models of the pelvis and femur reconstructed from the CT data. Gender differences were found not only in joint orientation, including anteversion and inclination of the acetabulum and femoral neck anteversion, but also in the shape around the joint, including the acetabular rim and the femoral neck. This ROM study also showed gender differences in all four standard directions. In conclusion, significant gender differences were observed in the acetabular and femoral morphology, which led to significant gender differences in ROM until bony impingement. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:333–339, 2011     

Dislocation after total hip arthroplasty: implant design and orientation

Implant design and positioning are important factors in maintaining stability and minimizing dislocation after total hip arthroplasty. Although the advent of modular femoral stems and acetabular implants increased the number of head, neck, and liner designs, the features of recent designs can cause intra-articular prosthetic impingement within the arc of motion required for normal daily activities and thus lead to limited motion, increased wear, osteolysis, and subluxation or dislocation. Minimizing impingement involves avoiding skirted heads, matching a 22-mm head with an appropriate acetabular implant, maximizing the head-to-neck ratio, and, when possible, using a chamfered acetabular liner and a trapezoidal, rather than circular, neck cross-section. Computer modeling studies indicate the optimal cup position is 45 degrees to 55 degrees abduction. Angles <55 degrees require anteversion of 10 degrees to 20 degrees of both the stem and cup to minimize the risk of impingement and dislocation.