Acetabular Cup Orientation Research Articles

Resolving inconsistencies in defining the target orientation for the acetabular cup angles in total hip arthroplasty

Background Dislocation following total hip arthroplasty is a major complication and malorientation of the acetabular cup is one of the primary factors affecting dislocation. Different conventions used to describe the cup orientation produce significant variations in the recommendations for correct positioning, which in turn make it difficult for clinicians to properly interpret and apply previously reported studies. Methods We examined nine articles presenting recommendations for the range of target orientations of the acetabular cup to minimize the risk of dislocation (referred to as the ‘safe zone’). Those studies included five ways to define the cup orientation and two methods to define the reference frame. We converted those recommendations to a single representation based on the radiographic angles expressed in the pelvic frame reference. Findings After conversion, the mean recommended anteversion angle was shifted downward by 5° ( P < 0.01). Also, the target orientation recommendations became more consistent, especially for the anteversion angles where the standard errors of the upper and lower limits were reduced by 61% ( P = 0.02) and 23% ( P = 0.04), respectively. Interpretation The choice of reference frame and the definition for acetabular cup orientation angles can have a significant effect on the target orientation for the acetabular cup. Recommendations for the target orientation should always explicitly state which reference frame and angle definition is being used. The averaged recommendation of the studies assessed here is 41° inclination and 16° anteversion in radiographic angles or 39° inclination and 21° anteversion in operative angles, both expressed in the pelvic reference frame.

Risk Factors for Ventral Luxation in Canine Total Hip Replacement

To identify risk factors associated with ventral luxation (VL) of canine total hip replacement (THR), and outcome. Retrospective study. Dogs (n=602) that had THR (563 cemented, 35 cementless, 4 hybrid). Dogs (1999-2004) with VL after THR were compared with dogs with uncomplicated THR. Data included signalment, body weight, diagnosis, implant size, acetabular cup orientation, and femoral displacement ratio (FDR). VL was diagnosed in 11 (1.8%) dogs after primary THR. Including 2 other dogs that had VL after the study period, 10 (77%) of 13 dogs had VL within 7 days of surgery. Risk factors for VL included Saint Bernard-type dogs (P=.0001), short neck extension (P=.0005), and high angle of lateral opening in other breeds (P=.018). There were trends toward higher risk of VL with lower FDR in Saint Bernard types (P=.060), and with cementless implants (P=.061). Twelve dogs had revision arthroplasty that was successful in all cementless and 2 cemented VL cases. Five dogs had recurrent VL and a poor outcome. VL is generally an early complication of THR, with no single common risk factor identified. Saint Bernard types and short neck extensions are associated with increased risk of VL. Poor cup orientation is a determining factor for VL in some dogs, but a protective orientation of the acetabular cup was not found. Revision is successful in most dogs, but recurrent VL merits a guarded prognosis.

Reliable Acetabular Cup Orientation With a New Gravity-Assisted Guidance System

Acetabular cup orientation is a key factor determining hip stability, and standard mechanical guides have shown little help in improving alignment. An in vitro study was carried out to compare the accuracy and precision of a new gravity-assisted guidance system with a standard mechanical guide. Three hundred ten cups were impacted by 5 surgeons, and the final cup orientation was measured. With the new guide, the average error in anteversion was 0.4°, compared with 10.4° with the standard guide and 0.3° and −4.7°, respectively, for abduction angles. The average time required for orienting the cups was similar for both guides. The accuracy and reproducibility obtained with the new guide were better ( P < .0001). These good results would require a clinical validation.

Factors Influencing Cup Orientation in 500 Consecutive Total Hip Replacements

We sought to establish the percentage of acetabular components used in total hip arthroplasties that were located outside a presumed safe range of cup orientation. Data were analyzed to assess whether dislocation in this series was different inside and outside that presumed safe zone. We also asked whether acetabular cup orientation depended on patient body mass index, the amount of preoperative acetabular head coverage, the surgeon, or the use of minimally invasive technique. We assessed cup orientation in 500 total hip arthroplasties performed at one institution. Of these 500 total hip arthroplasties, 400 were done using conventional approaches whereas mini-incisions were used in 100. We found 19.8% of cups were oriented outside the presumed safe range for inclination, and 11.2% of cups were oriented outside the presumed safe range for anteversion. Dislocation was not greater in the group with inclination and anteversion outside the presumed safe. Cup orientation was influenced by pre-operative acetabular head coverage, the surgeon, and minimally invasive technique, but not body mass index. Cup variability was greater than expected. It was not confined to one surgeon, but to the entire group of surgeons experienced in doing total hip replacements. Variability points toward continuous refinement in surgical technique and instrumentation to promote consistency in cup placement. Prognostic study, Level IV (case series).

Measuring anatomical acetabular cup orientation with a new X-ray technique

Proper alignment of the acetabular cup component is one of the most important requisites for a successful long-term outcome in total hip replacement. However, measurement and indication of cup orientation in an anatomical pelvic reference system is very difficult. We propose a new C-arm-based X-ray technique for determining the values for inclination and anteversion of the acetabular cup component. The proposed method is validated by computer simulation and sources of error are evaluated. The method predicts an accuracy of better then 5° for determination of anteversion of the cup.

Measuring anatomical acetabular cup orientation with a new X-ray technique

Proper alignment of the acetabular cup component is one of the most important requisites for a successful long-term outcome in total hip replacement. However, measurement and indication of cup orientation in an anatomical pelvic reference system is very difficult. We propose a new C-arm-based X-ray technique for determining the values for inclination and anteversion of the acetabular cup component. The proposed method is validated by computer simulation and sources of error are evaluated. The method predicts an accuracy of better then 5° for determination of anteversion of the cup.

Measurement of acetabular cup orientation in total hip arthroplasty using a volume registration technique

Objective. To compare the in vitro stability of two cemented hip stem designs: Stem I was a collarless, double-tapered, highly polished implant; Stem II had a collar and matt finish.Background. Stability of the femoral component of a hip implant is important for its long-term clinical success. Excessive migration or cyclic motion can increase the risk of early implant failure.Methods. The stems were implanted in paired human cadaver femurs, and custom-designed micromotion sensors were used to measure three-dimensional motions of the stems at proximal, middle and distal locations during simulated in vivo loading cycles.Results. This study found that despite ‘rigid’ fixation, cemented stems exhibit detectable motions under a limited number of cycles of simulated physiologic loads. At four times the donor body weight, Stem I showed a subsidence of 90 μm, compared to 25 μm of Stem II (P<0.05). In contrast, the proximal end of Stem II exhibited greater cyclic motions in the medial–lateral direction (P<0.05).Conclusions. The different motion patterns could be due to the design differences, such as surface finish and geometry.RelevanceImplant design is an important factor related to the behavior of the cement/bone interface and the overall success of the implant. This study compares in vitro micromotion of two cemented femoral prostheses with differing proximal designs.

Stability of acetabular axis after total hip arthroplasty, repeatability using CT and a semiautomated program for volume fusion.

To validate a CT method for detecting changes in acetabular cup orientation after THA. 26 CT examinations were obtained from a pelvic model with an uncemented acetabular cup. The model position was altered between acquisitions, but the cup axis angle vis-à-vis the pelvis was maintained. Data sets were combined into 37 pairs, each containing a unique positioning error. The pelvi in different examinations were fused, creating transformed volumes. Landmarks corresponding to the cup before and after fusion were placed interactively by two independent examiners. The orientation of the acetabular axis was calculated for each volume and compared across volumes. Before fusion the mean angle error between the acetabular axes was 4.17 degrees (SD +/- 1.95 degrees ). After fusion the mean angle error was 0.36 degrees (SD +/- 0.17). The 95% repeatability limits were below 0.7 degrees. There was no significant interobserver difference. Analysis of the cup landmarking pattern by condition numbers and individual landmark errors showed stability. Non-invasive fusion of CT volumes and a stable landmarking pattern for the acetabular cup outperforms routine plain radiography in detecting changes in the orientation of the acetabular axis over time. The method delivers both visual and numerical output and could be used in clinical practice.

Stability of Acetabular Axis after Total Hip Arthroplasty, Repeatability Using CT and A Semiautomated Program for Volume Fusion

To validate a CT method for detecting changes in acetabular cup orientation after THA. 26 CT examinations were obtained from a pelvic model with an uncemented acetabular cup. The model position was altered between acquisitions, but the cup axis angle vis-à-vis the pelvis was maintained. Data sets were combined into 37 pairs, each containing a unique positioning error. The pelvi in different examinations were fused, creating transformed volumes. Landmarks corresponding to the cup before and after fusion were placed interactively by two independent examiners. The orientation of the acetabular axis was calculated for each volume and compared across volumes. Before fusion the mean angle error between the acetabular axes was 4.17 degrees (SD +/- 1.95 degrees ). After fusion the mean angle error was 0.36 degrees (SD +/- 0.17). The 95% repeatability limits were below 0.7 degrees. There was no significant interobserver difference. Analysis of the cup landmarking pattern by condition numbers and individual landmark errors showed stability. Non-invasive fusion of CT volumes and a stable landmarking pattern for the acetabular cup outperforms routine plain radiography in detecting changes in the orientation of the acetabular axis over time. The method delivers both visual and numerical output and could be used in clinical practice.

Kinematics, kinetics, and finite element analysis of commonplace maneuvers at risk for total hip dislocation

Dislocation remains a disturbingly frequent complication of total hip arthroplasty (THA). Over the past several years, increasingly rigorous biomechanical approaches have been developed for studying dislocation, both experimentally and computationally. Realism of the input motion challenge data has lagged behind most other aspects of this body of work, and anterior dislocation maneuvers remain unstudied. To enhance realism of biomechanical studies of dislocation, motion data are here reported for ten THA-aged subjects, each repeatedly performing seven maneuvers known to be dislocation-prone. An optoelectronic motion tracking system and a recessed force plate captured the kinematics and ground reaction forces of these maneuvers. Using an established inverse dynamics model to estimate hip joint loading, 354 motion trials were evaluated using an existing finite element model of THA dislocation. Worst-case-scenario THA constructs were simulated (22 mm femoral head, acetabular cup orientations at the limit of the accepted safe zone), in order to deliberately induce impingement and dislocation. The results showed a high incidence of computationally predicted dislocation for all movements studied, but also that risk was very maneuver-dependent, with patients being six times more likely to dislocate from a low-sit-to-stand maneuver than from stooping. These new motion data hopefully will help facilitate systematic efforts to reduce the incidence of dislocation.

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° and 45° and cup anteversion between 0° and 10°. Cup anteversion angles greater than 20° and cup angles of inclination greater than 45° significantly limited internal and external rotation, particularly for hip flexion greater than 60°. 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.

Radiographic assessment of acetabular prosthesis anteversion: an experimental study.

Two trigonometric methods used to assess acetabular cup orientation radiographically derive the angle of anteversion differently and yielded statistically significantly different results when compared using a laboratory model of the pelvis. The need for standardisation is suggested.

A Comparison of the Anterolateral, Transtrochanteric, and Posterior Surgical Approaches in Primary Total Hip Arthroplasty

For evaluation of both the anterolateral and posterior approaches to total hip arthroplasty as alternatives to the transtrochanteric approach, 269 replacement procedures in 237 patients were reviewed. Follow-up periods ranged from 24 to 71 months (average, 38.2 months). The anterolateral approach was used in 91 hips, the transtrochanteric approach in 136, and the posterior approach in 42. Dislocation was four times more common (p less than .05) in the posterior approach (9.5%) than in the other two (2.2% each). Seven of the nine dislocations occurred in patients older than 65 years of age, and all four dislocations in the posterior approach group occurred in patients over 65. Heterotrophic bone was three times more common (p less than .10) in posterior and transtrochanteric approaches than in the anterolateral approach. Trochanteric bursitis was twice as common (p less than .05) in the transtrochanteric approach than in the other two. Roentgenographic evaluation of femoral component positioning, width of medial calcar cement, and acetabular cup orientation showed no significant differences between the three surgical approaches. It is apparent that the anterolateral surgical approach (modified Watson-Jones) provides a satisfactory alternative to trochanteric osteotomy in the elderly patient who requires total hip arthroplasty.

Radiographic Assessment of Acetabular Cup Orientation

An acetabular cup with a 270 degree rim wire has been designed to assist in the radiographic assessment of acetabular cup orientation in total hip arthroplasty. A study was conducted analyzing the component in various positions of lateral opening and version with three different X-ray-beam centering positions. The 270 degree acetabular rim wire allowed for easy and accurate assessment of component orientation when the pelvis was level and the central ray of the X-ray beam was centered over the hip.

Can Wear in Total Hip Arthroplasties Be Assessed from Radio graphs?

The validity of wear measurements from total-hip radiographs was assessed using a wear simulator which consisted of a total hip prosthesis mounted on a Plexiglas orientation jig. Wear was mesured by a micrometer device and uni- and duoradiographic methods of wear measurement were used with a variety of acetabular cup orientations. In the initial calibration studies the duoradiographic technique proved to be more accurate. The source of the inaccuracies of the uniradiographic method was determined analytically by means of a geometrical model. Forty-inch or 72'' source-to-object film-distances provided similar accuracy and the errors in wear measurement varied from --0.1 +/- 0.4 mm (2 S.D.) to 0 +/- 0.9 mm for wear magnitudes of 1 and 5 mm respectively, regardless of cup position. In subsequent calibration studies the previous radiographs were compiled to represent seven hypothetical patients at various stages of follow-up whose prostheses exhibited between 1 and 5 mm wear. Clinicians assessed the degree of wear, using both the uni- and duoradiographic techniques, under supervision but unaware of the true position and orientation of the wear track. The resulting errors were of the same order as the wear magnitudes being measured. The location of the measurements on the radiographs was then restricted to approximately the true wear axis but again the errors were so large that it was still impossible to make any valid wear assessments. The calibration runs were therefore terminated at this point and it was concluded that wear measurements could not be made from clinical radiographs.