HIP

ORTHOPEDICS June 2011;34(6):127.

In Vivo Assessment of Total Hip Femoral Head Separation from the Acetabular Cup During 4 Common Daily Activities

by Thomas J. Blumenfeld, MD; Diana A. Glaser, PhD; William L. Bargar, MD; Glen D. Langston, BS; Mohamed R. Mahfouz, PhD; Richard D. Komistek, PhD

Abstract

In vivo video fluoroscopies of well-functioning total hip arthroplasties (THA) have shown that femoral head separation from the medial articular bearing surface occurs during gait. Other activities may cause the same phenomenon. We examined this while patients performed the following 4 activities of daily living: pivoting to each side in stance, shoe tying, sitting down, and standing up. Ten healthy patients (5 men, 5 women, average age 66 years) all 1 year or more after cementless THA performed for degenerative arthritis, with Harris Hip Scores =90, were studied. Each patient performed the activities of daily living while data was captured using video fluoroscopy. Based on previously reported criteria, femoral head separation (the femoral head sliding lateral to the acetabular liner) was determined to be reliably predicted if the distance between the femoral head and acetabular cup was =0.5. Results showed that the greatest femoral head separation occurred during the pivoting activity (mean, 1.53 mm; range, 0.00–3.34 mm; SD, 1.05 mm). The separation values identified during pivoting occurred at the extremes of internal or external rotation for all patients. The other 3 activities showed lower separation distances. Separation during the pivoting activity exceeded the reported separations occurring during walking. This finding was seen in a small group of patients, and the data should be interpreted with caution. We conclude from this study that the evaluation of gait alone may not be sufficient to accurately assess femoral head separation occurring during activities of daily living for healthy, active patients.

Drs Blumenfeld and Bargar are from Sutter General Hospital, University of California Davis and Dr Glaser is from Rady Children’s Hospital of San Diego, California; Mr Langston is from Wright Medical Technology, Inc, Arlington, and Drs Mahfouz and Komistek are from the University of Tennessee, Knoxville, Tennessee.

Dr Blumenfeld is a consultant for DePuy. Drs Glaser, Mahfouz, and Komistek and Mr Langston have no relevant financial relationships to disclose. Dr Bargar is a consultant for Curexo.

Funding for the data acquisition and analysis was obtained from DePuy, a Johnson & Johnson Company. The funding allowed for obtaining data on 10 patients.

Correspondence should be addressed to: Thomas J. Blumenfeld, MD, 1020 29th St, Sacramento, CA 95616 (tblumenfeld@jointsurgeons.com).

Posted Online: June 14, 2011

The postoperative outcomes of total hip arthroplasty (THA) patients have been broadly studied through the use of patient questionnaires, ¹ instrumented implants, ² and kinetic and kinematic evaluations. ^3–7The majority of these studies have focused on the evaluation of implant performance during gait since walking is the predominant weight-bearing activity occurring in the daily lives of most individuals. The performance of hip prostheses during other common activities of daily living may be important to analyze. Other activities may lead to higher femoral head separations (the femoral head sliding lateral to the acetabular liner) than present during gait; therefore, a variety of movements must be studied to gain a full understanding of the conditions occurring at the prosthetic hip joint.

In vitro methods have been developed to test the long-term endurance of implants, taking into account a variety of activities of daily living. ⁸ These studies, which are of value for preclinical evaluations, do not analyze the performance of implants in a true in vivo environment. Therefore, in vivo analysis of hip joint prostheses is required in an attempt to understand and define prosthetic functioning. Instrumented implants have been used to analyze hip contact forces under in vivo conditions during various activities of daily living. ^9–11 Hodge et al ¹² showed that while the observed force on the implant during the ascension of stairs was higher than during walking, it was not as high as rising from a chair. In contrast, a separate telemetric study revealed no substantial difference between the peak resultant force (2.6× body weight) during gait and ascension of stairs. ¹⁰

Previous in vivo studies have determined that femoral head sliding within the acetabular cup does occur in THA patients, and that the magnitudes of femoral head separation are higher for abduction/adduction activities than for gait. ^4,13 The subsequent impact following femoral head separation from the liner leads to increased loading conditions at the bearing surface interface, especially superolaterally, which may lead to increased wear at the bearing surfaces of the implants. ^4,6,13–15 To date, no studies exist focusing on the fluoroscopic evaluation of in vivo hip kinematics and femoral head separation for a variety of activities of daily living.

The objective of the present pilot study was to obtain and evaluate the in vivo femoral head separations of 10 well-functioning THA patients while performing a variety of common activities of daily living not yet studied in detail: pivoting, tying a shoe, standing up and sitting down, both performed with and without the aid of handrails.

Materials and Methods

Patients

Ten patients implanted with a well functioning THA were examined. Five men and 5 women were analyzed under in vivo, weight-bearing conditions using video fluoroscopy. All patients received a similar cementless total hip prosthesis with a Summit femoral stem, a Pinnacle acetabular cup, a metal 36-mm diameter femoral head, and a Marathon cross-linked polyethylene liner (all implants Depuy, Johnson and Johnson Company, Warsaw, Indiana). All surgeries were performed by 2 of the authors (T.J.B., W.L.B.), both using a similar posterolateral approach with repair of the capsule and short external rotators. The average patient age was 66.1 years (range, 53–77 years). Body Mass Index values revealed 3 patients in the normal range (Body Mass Index, <25) and 7 patients in the overweight or obese category. The patient demographics overlapped the age and Body Mass Index distribution of the typical THA population. All patients were diagnosed with degenerative arthritis, 2 patients had acetabular dysplasia, and 1 patient had a stage 1 protrusio deformity.

Only THA patients with excellent clinical results (Harris Hip Scores >90 points (mean, 96 points; range, 90–100 points) presenting no functional deficits, an absence of generalized inflammation, and negligible chronic pain were included in the study. ¹⁶ Each patient was implanted with a unilateral THA and could independently abduct their operated hip against gravity without difficulty. None of the patients walked with a detectable limp. No patient sustained a hip dislocation or reported hip subluxation. Radiographic measurements of limb length were obtained on all patients. None had shortening of the analyzed limb in comparison to the unoperated side. The femoral offset was restored, and the acetabular component was oriented within the safe zone position for all THA patients. ¹⁷ The average duration of postoperative follow-up at the time of analysis was 13.1 months (10.3–20.5 months).

Activities

Each patient performed 4 different activities to evaluate the femoral head separation: pivot, shoe tie, sit-down and stand-up (both with and without handrails). The pivot activity was defined as turning of the upper body while leaving both feet firmly planted on the ground starting in the most internally-rotated position possible (upper body twisted toward the side with the hip implant) and then turning the upper body to the most externally-rotated position possible (upper body twisted away from the side with the hip implant). The shoe tie exercise consisted of a patient bending the upper body forward while sitting in a chair, from an erect seated position to a position nearly parallel to the floor while reaching between the knees (simulating the act of tying a shoelace). Sit-down exercises were executed by having a patient slowly sit in a chair (seat height 18 in) with arms from an upright position. Stand-up was then the opposite; the patient rose from sitting in a chair to an erect, standing position. Patients were allowed to hold the arms of the chair for stability if they wished.

Data Collection

Preliminary screening questionnaires were used for evaluation of the Harris Hip Score. Two cameras were used in conjunction with a single-plane fluoroscopy unit to capture the in vivo weight-bearing movement of the implants in the hip joint and the corresponding leg during each activity. The activity to be performed was modeled for the patient by 2 of the authors (T.J.B., W.L.B.). The patient was allowed to perform several trials of the activity prior to data acquisition. For all activities, patients were asked to keep equal weight on their limbs; validation of this via use of a force plate was not performed. Data was obtained from a single repetition of the activity for each patient. Patient data-sets were processed and interpreted using a 3-dimensional ^18,19 registration process alongside MATLAB (The MathWorks, Inc, Massachusetts), programs authored by the Center for Musculoskeletal Research. ¹⁹ For each patient, fluoroscopic video frames were digitized to specific time intervals according to the requirements for analysis of each activity, which ranged from 4 frames needed for shoe tie, sit-down, and stand-up to 5 frames needed for the pivot exercise. Both femoral and pelvic 3D translational and rotational kinematics were gathered for analysis. Based on the transformation matrices obtained for each individual body, relative motions of the femur and pelvis were calculated and then used to determine the distance between the center of the femoral head and the acetabular cup components. Application of this measurement allowed a diagnosis concerning whether or not sliding of the femoral head from the acetabular cup (separation) had occurred (Figure ). ^4,5

Figure 1:. Demonstration of the Overlay Method and of the Diagnosis of Separation (the Distance Between the Center of the Femoral Head and the Acetabular Component Is Denoted as Hip Joint Separation when >0.5 mm).

Error Analysis

An error analysis was previously published and confirmed the precision of the 3-dimensional model-fitting process. ^13,19 An error value of 0.5 mm was determined to be the threshold; therefore, femoral head sliding was reliably predicted if the distance between the femoral head and acetabular cup was >0.5 mm.

Results

Values representing the separation of the femoral component from the acetabular cup have been calculated for each activity and reported in millimeters. In the present study, we found that the separation values differed for the 4 activities (Figure ). The highest average hip separation was observed during the pivot activity with a mean of 1.53 mm (range, 0.00–3.34 mm; SD, 1.05 mm). Corresponding lowest separation values occurred while performing the stand-up activity, with an average of 0.69 mm (range, 0.00–1.60 mml; SD, 0.46 mm).

Figure 2:. Quartile Boxplot of Separation Results from 4 Activities.

Using a threshold of separation significance of 0.5 mm, observations of significant separation occurred in 9 of 10 (90%) patients during the pivot and sit-down exercises, which represented the highest incidence in the study. Conversely, the stand-up exercise showed only 6 of 10 (60%) patients having a separation >0.5 mm (Figure ). High hip separation incidence persisted during pivot when analyzing separation at a threshold of 1.0 mm (60%); considerably less incidence was found in shoe tie and sit down and was nearly absent in stand up (Figure ).

Figure 3:. Incidence of Separation >0.5 mm per Activity. For the Pivot Activity (PI, Blue) and Sit down Activity (SDOWN, Green), 9 of 10 Patients Exhibited Separation Greater than 0.5 mm. For the Shoe Tie Activity (SHOE, Red), 8 of 10 Patients Exhibited Separation >0.5 mm. For the Stand up Activity (SUP, Purple), 6 of 10 Patients Exhibited Separation >0.5 mm.

Figure 4:. Incidence of Separation >1.0 mm per Activity. For the Pivot Activity (PI, Blue), 6 of 10 Patients Exhibited Separation >1.0 mm. For the Shoe Tie Activity (SHOE, Red), 4 of 10 Patients Exhibited Separation >1.0 mm. For the Sit down Activity (SDOWN, Green), 3 of 10 Patients Exhibited Separation >1.0 mm. For the Stand up Activity (SUP, Purple), 1 of 10 Patients Exhibited Separation >1.0 mm.

Distinct trends were observed for each activity by evaluating the calculated separation values during the execution of the motion. In our analysis of the pivot activity, maximal separation occurred during internal rotation of the hip in 80% of patients. The highest separation values were seen at the extreme of rotation, while little or no separation occurred in the neutral position. For 2 of the 10 patients, lower separation values were identified as the hip rotated externally through the prescribed motion.

All 10 patients showed the greatest separation in the latter half of the shoe tie activity, during which the upper body is most bent forward. For the sit-down exercise, half of the patients (5/10) demonstrated maximal separation during the first half of the movement, and the other patients exhibited maximums during the latter half of the motion. Overall, a trend of decreasing separation values started from the beginning (standing) to the end (sitting) of the movement. For the stand-up exercise, half of the patients (5/10) demonstrated maximal separation during the first half of the activity, while the other patients exhibited maximums during the latter half of the motion. Contrary to the sit-down exercise, the opposing trend of increasing separation was apparent throughout the motion (from sitting to standing).

Discussion

Past fluoroscopic studies on THA separation are limited and have focused primarily on analysis during gait or abduction/adduction. ^4,14 Using in vivo fluoroscopic analysis to evaluate other motions commonly encountered during a typical day has not yet been performed. Kinematics obtained from video fluoroscopy has been found to be consistently accurate to within 0.5 mm. ^18,19 This process has been successfully used to analyze many different joints and implant systems in vivo. ^5,20–22

Previously, separation was found to occur in gait and abduction/adduction activities. ^6,13,14,18 Further analyses were necessary to determine if separation occurred during other activities of daily living and if the type of activity affected the incidence and magnitude of hip separation. We found a high incidence and magnitude of separation for the activities analyzed in the present study. In observed cases of separation between the femoral head and acetabular cup, contact area between the 2 components is reduced. In this case, a separation of the femoral head from the medial articular surface leads to a smaller superolateral region of contact. Therefore, the femoral head may pivot on the peripheral rim of the liner when separation values are extreme. This lessens the articular contact area, causing higher exerted pressures on the articular surfaces, potentially leading to increased wear of the components.

In comparison with the past studies, our findings during the pivot activity exceed the common separations encountered in walking. Average separation during pivot was 1.5 mm higher than the previously reported 1.2 mm of separation encountered during gait. ¹⁴ Lombardi reported a separation incidence of 100%, similar to the 90% observed for the pivot motion. ¹⁴ When comparing the pivot separations to separations previously reported for abduction/adduction analysis, the pivot exercise demonstrated lower overall separation values. Separation averages for abduction/adduction are reported as 2.4 and 3.3 mm, respectively, higher than the 1.5 mm observed for pivot motions within this study. ^13,14 We believe that the separations seen during the pivot activity are important, as discussed below.

The other activities, shoe tie, sit down, and stand up, all exhibited lower separation values than the pivot motion. When comparing these 3 activities to previously reported gait separation, the magnitudes and incidences of separation are considerably lower. From these results, we can conclude that examination of shoe tie, sit down, and stand up is not as important as the consideration of pivot, gait, and abduction/adduction activities when focusing on the extremes of hip joint activity during daily usage.

This study has 4 significant limitations:

1. We examined a small group of patients, and all patients had well-functioning THAs. Our results should not be used as normative values until a larger group of patients are studied. More importantly, our results may not apply in patients with hip instability or abductor weakness; we would suspect that the separations in these conditions would be higher.

2. Other kinematic features, such as the amplitude and direction of angular rotation of the femoral head within the acetabular cup, were not studied.

3. We have chosen to report our findings relative to previously reported studies on femoral head separation, and the patient populations may not be comparable.

4. The separations seen during pivoting demonstrated in this study, occurring during 2-legged stance, may not represent those occurring in conjunction with gait.

We conclude from this study that the evaluation of gait alone may not be sufficient to accurately assess the range of separation values encountered in daily life for healthy, active patients. Of the investigated activities presented herein, separation value averages for the pivot motion were greater than those found previously for gait. As selected patients are now being allowed to run after THA, if the separations we have demonstrated with pivoting exist at the higher velocities encountered with running, the bearing separation and subsequent joint relocation forces may be detrimental to implant longevity. The upper boundaries of separation values in hip arthroplasty in both well-functioning and poorly-functioning THAs require further investigation.

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