Background: Inadequate soft-tissue tension in total hip arthroplasty is regarded as one cause of dislocation or abductor muscle weakness. The purpose of the present study was to assess how the postoperative discrepancy in limb offset (consisting of both femoral offset and acetabular offset) affects soft-tissue tension compared with other factors among patients with unilateral hip disease undergoing total hip arthroplasty.
Methods: A total of 89 consecutive patients underwent mini-incision total hip arthroplasty involving an anterolateral or posterior approach and with use of a computer navigation system. Soft-tissue tension was measured by applying traction amounting to 40% of body weight with the joint positioned at 0°, 15°, 30°, and 45° of flexion. The separation between the cup and the prosthetic head was measured using the navigation system.
Results: The cup-head separation differed significantly for varying angles of flexion (p < 0.001), with the greatest distance noted at 15° of flexion (mean and standard deviation, 11 ± 5 mm). Stepwise multiple regression analysis showed that postoperative limb-offset discrepancy, an anterolateral approach, and preoperative abduction range of motion were correlated with the cup-head separation at 15° of flexion. Postoperative limb-offset discrepancy was negatively correlated with the cup-head separation at 0°, 15°, and 30° of flexion.
Conclusions: Postoperative limb-offset discrepancy significantly affected the soft-tissue tension in total hip arthroplasty at varying degrees of flexion. This indicated that it is important to restore normal limb offset without overlengthening to obtain adequate soft-tissue tension in total hip arthroplasty.
Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.
1Departments of Orthopaedic Surgery (M.T. and T.S.) and Orthopaedic Medical Engineering (T.N. and N.S.), Osaka University Graduate School of Medicine, Osaka, Japan
E-mail address for M. Takao: firstname.lastname@example.org
Inadequate soft-tissue tension in total hip arthroplasty is regarded as a cause of dislocation or abductor muscle weakness1,2. The femoral head-neck length and offset are the parameters that surgeons can modify intraoperatively to obtain adequate soft-tissue tension. A limb traction test, the shuck test, is usually used to evaluate soft-tissue tension, whereby traction is applied to the limb in the distal direction, and the separation between the femoral head and the cup is measured3. While some have emphasized the importance of restoration of femoral offset to optimize soft-tissue tension in total hip arthroplasty1,2, it is unclear how important other factors, such as preoperative range of motion, change in limb length, and surgical approach, are to determining the soft-tissue tension in total hip arthroplasty.
The purpose of the present study was to assess how the postoperative discrepancy in limb offset (consisting of both femoral offset and acetabular offset) affects soft-tissue tension compared with other factors in the treatment of patients with unilateral hip disease undergoing total hip arthroplasty with use of a computer navigation system.
Materials and Methods
Institutional review board approval was obtained for this prospective cohort study. Eighty-nine consecutive patients with unilateral hip disease underwent mini-incision, cementless total hip arthroplasty with use of a computed tomography (CT)-based navigation system (Hip Navigation, version 1.1; Stryker Navigation). Patients were in the lateral position. A posterior approach was used for 57 patients and an anterolateral approach, for 32 patients. Cementless anatomical stems (CentPillar; Stryker) or cementless tapered stems (Accolade TMZF; Stryker) were used. The neck-shaft angle of both was 127°. Cementless cups (Trident; Stryker) with polyethylene liners without elevated rims (Crossfire; Stryker) were used in all patients. The diameter of the prosthetic head was 32 mm in 2 patients, 36 mm in 66, 40 mm in 19, and 44 mm in 2. Preoperative patient demographics and radiographic data are presented in Table I.
Preoperative CT images of each patient, from the level of the anterior superior iliac spine of the pelvis to the level of the femoral condyles, were made using a helical CT scanner (HiSpeed Advantage; GE Healthcare). The slice thickness was 1 mm and the pitch was 3 mm around the hip and knee. In the thigh area, the slice thickness was 3 to 10 mm and the slice pitch was 2 mm, to reduce radiation exposure.
The position and orientation of the cup and stem were planned using the preoperative planning module of the navigation system to minimize discrepancies in limb length and offset, which were determined according to the pelvic and femoral coordinate systems described below.
The pelvic coordinates were defined as follows: the anterior pelvic plane through the anterior superior iliac spines and the pubic tubercles was oriented according to the tilt of the plane when the patient was supine on the CT table; a line through the distal ends of the bilateral ischia was used to adjust the horizontal axis, and a line through the pubic symphysis and the center of the sacral body was used to adjust the anteroposterior axis4-6.
The femoral coordinates were defined as follows: the vertical axis referred to a line through the trochanteric fossa and the center of the knee, and the coronal plane was parallel to the posterior condylar plane through the posterior prominence of the greater trochanter and the posterior side of the femoral condyles4-6.
Limb length and offset were calculated on the basis of neutral positioning of the limb by aligning the femoral and pelvic coordinate systems. Limb length was defined as the distance from the line through the distal ends of the bilateral ischia to the center of the knee. Limb offset was defined as the distance from the line through the pubic symphysis and the center of the sacral body to the line through the trochanteric fossa and the center of the knee. The amount of required limb length and offset was calculated using the contralateral hip as a template.
Total hip arthroplasty was performed with the patient in a lateral position under general anesthesia. The dynamic reference markers were fixed to the iliac crest with 2 apex pins, each 4 mm in diameter. The dynamic reference markers for the femur were fixed, for the posterior approach, to the greater trochanter by a triangular plate with 3 small screws5 and for the anterolateral approach, to the femoral shaft with use of a Kirschner wire6. The cup and stem were implanted using the navigation system according to each preoperative plan. The orientation of both the cup and stem was measured using navigation with reference to the above-mentioned coordinate system. The amount of limb lengthening and lateralization and postoperative limb-length discrepancy and limb-offset discrepancy were also measured using the navigation system.
Measurement of Soft-Tissue Tension
After the placement of all components, an L-shaped metal plate fitted with a hook was fastened to the limb with inelastic bandages from the distal part of the thigh to the sole of the foot. A traction gauge (FB 500N; Imada) was attached to the hook. Traction amounting to 40% of body weight was applied with the joint positioned at 0°, 15°, 30°, and 45° of flexion, and the separation between the cup and the head was measured. The joint was adjusted so that the angle of internal or external rotation was kept within 5°.
To assess the effect of general anesthesia on the assessment of soft-tissue tension and postoperative changes of soft-tissue tension, the separation between the cup and the head was evaluated under fluoroscopy at 3 weeks, 3 months, 6 months, and 1 year after surgery for 20 patients in the group for which the posterior approach was used. Traction amounting to 40% of body weight was applied with the joint in the neutral position, and the separation between the cup and the head was measured on fluoroscopic images.
Scores on a numerical pain rating scale (NPRS)7 and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)8 were collected at 1 year postoperatively. The patients were asked to report pain intensity experienced in the previous week using the 11-point NPRS, with 0 representing no pain and 10 representing the worst pain imaginable.
An anteroposterior digital radiograph of the pelvis was made after surgery with the hips internally rotated 15° with the patient in the supine position. The position and alignment of the cup were measured using digital templating software (2D Template; Kyocera Medical). The vertical distance between the inter-teardrop line and the center of the prosthetic head was measured. The horizontal distance between the tip of the ipsilateral teardrop and the center of the prosthetic head (acetabular offset) was measured. The postoperative femoral offset was also measured; this was defined as the perpendicular distance from the center line of the proximal part of the femoral shaft to the center of the prosthetic head.
The Mann-Whitney U test was used for comparing continuous variables, and the Fisher exact probability test was used for comparing nominal variables between 2 groups. The Friedman test was used for comparing 3 or more pairs of groups. The linear association between 2 variables was assessed using the Spearman rank correlation test. P values of <0.05 were considered significant. The following 17 possible factors that might affect cup-head separation during the application of distal limb traction were evaluated using stepwise multiple regression analysis: age, sex, surgical approach, presence of developmental dysplasia of the hip (DDH), prosthetic head size, preoperative range of motion (flexion, extension, abduction, and adduction), amount of limb lengthening and lateralization, postoperative limb-length discrepancy and limb-offset discrepancy, stem anteversion, vertical and horizontal position of the cup, and postoperative femoral offset. The minimum required sample size for the multiple regression study was calculated to be 72, given the desired probability level of 0.05, the number of predictors in the model (17), the anticipated effect size (0.35), and the desired statistical power level of 0.8. A validation analysis for the multivariable linear regression models was conducted. The bootstrap method was used to randomly sample data from the original data set with replacement.
The intraoperative separation between the cup and the head during distal limb traction differed significantly at varying angles of flexion (p < 0.001), with the greatest distance at 15° of flexion (Fig. 1 and Table II). The cup-head separation was significantly larger for the anterolateral approach compared with the posterior approach at 0°, 15°, and 30° of flexion (Figs. 2 and 3 and Table II).
The stepwise multiple regression analysis showed that the postoperative limb-offset discrepancy was negatively correlated with the cup-head separation at 0°, 15°, and 30° of flexion, while the amount of limb lateralization did not correlate with the separation at any degree of flexion (Table III). The amount of limb lengthening was negatively correlated with the cup-head separation at 0° and 30° of flexion, while postoperative limb-length discrepancy was not correlated with the separation at any degree of flexion.
An anterolateral approach was positively correlated with the cup-head separation at 0° and 15° of flexion. Preoperative extension range of motion was positively correlated with the cup-head separation at 0° of flexion, and preoperative range of motion in abduction was positively correlated with the cup-head separation at 15° of flexion. Female sex was positively correlated with the cup-head separation at 30° of flexion. No variables were found to affect the cup-head separation at 45° of flexion. In the validation analysis for the multivariable linear regression models, the maximum optimism (defined as true performance minus apparent performance in a prediction model) was 4.8%, which indicated good calibration.
The NPRS score and the WOMAC subscale scores at 1 year of follow-up are shown in Table II. The intraoperative cup-head separation at 0° and at 15° of flexion was negatively correlated with the WOMAC physical function subscale score (Spearman correlation coefficient of −0.33 and −0.35, respectively). There was no dislocation during the 1 year of follow-up.
The intraoperative cup-head separation at 15° of flexion was correlated with the postoperative cup-head separation measured 3 weeks after surgery (Table IV). The mean ratio of postoperative cup-head separation to intraoperative cup-head separation at 15° of flexion was 0.32 at that time. During the follow-up period, the cup-head separation significantly decreased from 3 weeks to 1 year (p = 0.034) (Table IV and Fig. 4).
Restoration of the normal femoral offset in total hip arthroplasty is reported to be important in order to obtain good abduction muscle strength9-13 and gait function14. It has been unclear how postoperative limb offset affects soft-tissue tension around total hip arthroplasty components. In the present study, postoperative limb-offset discrepancy significantly affected the cup-head separation during distal limb traction at 0°, 15°, and 30° of flexion. This indicated that it is important to restore the normal limb offset to obtain adequate soft-tissue tension in total hip arthroplasty at varying degrees of flexion. The limb offset consists of acetabular offset and femoral offset. In the present study, the horizontal distance between the tip of the ipsilateral teardrop and the center of the prosthetic head corresponded to the acetabular offset. The perpendicular distance from the center line of the proximal part of the femoral shaft to the center of the prosthetic head corresponded to the femoral offset. Neither the acetabular offset nor the femoral offset determined the soft-tissue tension by themselves. We consider that global limb offset has a greater effect on soft-tissue tension than either of the others independently because it reflects the relative horizontal positioning between the pelvis and the femur.
Postoperative limb-length discrepancy was not correlated with the cup-head separation during distal limb traction at any flexion position. The amount of limb lengthening was negatively correlated with the cup-head separation at 0° and 30° of flexion. This indicates that, if surgeons prioritize soft-tissue tension during surgery, the limb tends to be overlengthened. Naito et al. and Sathappan et al. also reported that uneven limb length can easily result when the shuck test is used as a measure of limb-length discrepancy15,16.
Several surgical approaches with minimal incisions are used in total hip arthroplasty, but their effect on soft-tissue tension is also unknown. In the present study, the cup-head separation was greater with the anterolateral approach at 0°, 15°, and 30° of flexion. According to the multivariable regression analysis, the anterolateral approach affected soft-tissue tension at 0° and 15° of flexion. In our procedure using the anterolateral approach, the iliofemoral ligament was released at the base of the femoral neck, which we believe resulted in wider cup-head separation at 0° and 15° of flexion. Further study would be necessary to explain why the anterolateral approach, which is commonly associated with a lower dislocation rate, was associated with greater laxity.
It is quite natural to assume that preoperative contracture of the affected hip joint determines soft-tissue tension in total hip arthroplasty. In the present study, preoperative extension range of motion was positively correlated with the cup-head separation at 0° of flexion. This indicated that a case with flexion contracture tended to show tight soft-tissue tension at 0° of flexion. In addition, preoperative abduction range of motion was positively correlated with cup-head separation at 15° of flexion. This indicated that a case with adduction contracture tended to show tight soft-tissue tension at 15° of flexion. Female patients tended to show lower soft-tissue tension at 30° of flexion. This might reflect sex differences in joint laxity or muscle volume17.
In order to assess the effect of intraoperative soft-tissue tension on clinical outcome, we evaluated the NPRS and WOMAC scores at 1 year after surgery. The intraoperative cup-head separation at 0° and 15° of flexion was negatively correlated with the WOMAC physical function subscale score. This result might indicate that excessive soft-tissue tension should be avoided for proper postoperative function of the hip.
Inadequate soft-tissue tension in total hip arthroplasty is regarded as a cause of dislocation, while it is uncertain whether attempting to achieve a stable soft-tissue envelope at all ranges of motion is desirable for the prevention of dislocation. There was no case of dislocation after surgery in the present study, but adequate soft-tissue tension for joint stability is still unclear. The dislocation rate of 0 in the present study might be because there were few outliers in cup and stem alignments and limb-length and offset discrepancy by using navigation. In the present study, the average cup-head separation was 11 ± 5 mm with a traction force of 40% of body weight at 15° of flexion. The maximum was 22 mm. These values are almost equal to or greater than the jumping distances at which dislocation occurred calculated by computer simulation in another study, in which the jumping distances were 12 mm for a 28-mm femoral head and 13 mm for a 32-mm femoral head at 40° abduction of the cup18. Additional study is required to determine what the ratio should be between the cup-head separation during the shuck test and jumping distance in order to contribute to joint stability.
It is also unclear how soft-tissue tension changes after the effect of anesthesia diminishes. The intraoperative cup-head separation at 15° of flexion was correlated with the postoperative cup-head separation at 3 weeks and 1 year after surgery. The ratio of the postoperative cup-head separation to the intraoperative cup-head separation was 0.32 at 3 weeks of follow-up, which reduced to 0.14 at 1 year of follow-up. Relatively lower soft-tissue tension in the early postoperative period might be a cause of dislocation during that period.
There were 3 limitations of the present study. The first limitation was measurement accuracy and reproducibility. Cup-head separation, limb length and offset, and stem anteversion were measured using the navigation system, so it is difficult to evaluate its measurement reproducibility. Kitada et al. reported that the average measurement accuracy of the navigation system was 0.6° for stem anteversion and 1.3 mm for limb length5. Thus, this measurement accuracy appears acceptable for evaluating cup-head separation by axial traction.
The second limitation was the level of traction force. There is no gold standard regarding the level of traction force in the shuck test. Traction amounting to 40% of body weight was applied to exclude the effect of body weight on the results. Preliminarily, to verify the level of traction force exerted by surgeons and their assistants, 21 orthopaedic surgeons were asked to pull a traction gauge horizontally with the same force that they would normally use when performing the shuck test, and the traction force applied was measured. The 21 surgeons included 7 experienced hip surgeons, 7 hip surgeons who had recently begun their practice, and 7 surgical residents. There was wide variation in the traction force among the surgeons, regardless of surgical experience. The average force was 24 ± 6 kg (range, 11 to 35 kg), which corresponded to 40% of the body weight of a patient weighing 60 kg. Thus, a traction force of 40% of body weight was chosen. We would like to emphasize that there were wide variations in traction forces among surgeons, which indicated that conventional assessment of soft-tissue tension is subjective and unreliable.
The third limitation was that a substantial number of the patients (58%) in the present study had DDH. Such patients have smaller femoral offset and larger limb-length discrepancy19, which could affect the soft-tissue tension in total hip arthroplasty. In the multiple regression analysis, however, the presence of DDH was not found to be a significant factor determining intraoperative soft-tissue tension. Additional investigation is necessary to ascertain whether the results of the present study can be applied to primary osteoarthritis.
In conclusion, postoperative limb-offset discrepancy significantly affected the cup-head separation during distal limb traction at varying degrees of flexion. This indicated that it is important to restore the normal limb offset without overlengthening of the limb to obtain adequate soft-tissue tension in total hip arthroplasty.
NOTE: The authors thank Hideki Yoshikawa, MD, PhD; Nobuo Nakamura, MD, PhD; Yuki Maeda, MD, PhD; and Makoto Hamawaki, MD, for their scientific advice and many insightful discussions.
Investigation performed at the Osaka University Graduate School of Medicine, Osaka, Japan
Disclosure: This study was carried out with the aid of a research grant from the Hip Joint Foundation of Japan. The funding source had no role in the study. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article.
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