Patients and Implant
Institutional review board approval was obtained, and all patients provided informed consent.
For the present study, only patients with unilateral osteoarthritis and a body mass index of <30 kg/m2 were selected. All patients were suitable candidates for a conventional total hip arthroplasty as well as for a minimally invasive hip replacement and did not favor either of the two methods in the preoperative interview conducted by the surgeon.
Seventy-one consecutive patients from the preoperative waiting list of the senior author (K.K.) were scheduled for unilateral primary total hip arthroplasty over a period of six months. The exclusion criteria included previous surgery on the evaluated hip, previous arthroplasties of other joints, a limb-length discrepancy of >0.5 cm, rheumatoid arthritis, and decreased mobility due to non-joint-related factors (e.g., Parkinson disease). Twenty-nine patients were excluded on the basis of these criteria. One patient refused to participate in the study, and another patient reported tolerable hip pain and did not want to undergo a total hip arthroplasty at that time. The remaining forty patients were randomized into two groups following a computerized protocol. Each patient received either the number 0 for the minimally invasive group or 1 for the standard group. Enrollment was stopped after twenty hips had been included in each group because of the fact that a laboratory gait evaluation including a dynamic electromyographical examination is a time-consuming method and because we found no published prospective gait-analysis study of this topic with higher sample sizes or type-2 error calculations to prove significance (Fig. 1-A).
In the first group (the minimally invasive group), patients underwent a primary unilateral total hip arthroplasty with use of a minimally invasive modified Watson-Jones approach. Those twenty patients were compared with a second group of twenty patients (the standard group) who received the same implants with use of a conventional transgluteal approach as described by Hardinge12. The demographic data on the patients are shown in Table I. No significant differences were found between the minimally invasive group and the standard group with regard to age at the time of surgery (p = 0.5), sex (p = 0.2), or body mass index (p = 0.9). All operations in the present study were performed by the senior author (K.K.). In all forty cases, the same standard type of implant was used, specifically, the cementless Alloclassic Variall system (Zimmer, Winterthur, Switzerland), a conical threaded cup in combination with a tapered straight stem. The articulating partners were ceramic-on-crosslinked polyethylene, metal-on-crosslinked polyethylene, or ceramic-on-ceramic.
Surgical Technique
In the minimally invasive group, the patient was positioned on the operating table in the supine position and both lower limbs were draped in a sterile fashion. An oblique skin incision measuring 8 to 10 cm was performed, extending distally from the anterior superior iliac spine and ending at the flare of the greater trochanter. After division of the subcutaneous tissue and fascia, the interval between the tensor fasciae latae and the gluteus medius was opened bluntly with the insertion of a finger. No muscle was split or detached with use of this technique. The acetabulum was prepared in a traditional fashion with use of standard reamers, with no difference from the standard group. For preparation of the femur, the distal half of the operating table was lowered approximately 30° and the involved lower limb was placed in external rotation under the contralateral lower limb. In this position, an elevating retractor was placed posterior to the greater trochanter to lever the femur out of the wound. Again, no muscle was detached. Further preparation of the femur was similar to that in the standard group and was performed with use of the same instruments.
In the standard group, the patient was placed in the supine position with only the involved lower limb draped. A lateral skin incision, approximately 12 cm in length, was performed. With use of the transgluteal approach as described by Hardinge, the fascia lata was split longitudinally and retracted. The distal portion of the gluteus medius and the proximal portion of the vastus lateralis were split in the direction of their muscular fibers to expose the joint. The gluteus medius was split approximately 3 cm proximal to the upper tip of the greater trochanter.
For preparation of the proximal part of the femur, the involved lower limb was positioned in external rotation over the contralateral lower limb. The insertion of the abductor muscles was partially released to allow adduction for better orientation. After final implantation, the split muscles and the fascia lata were closed completely.
The postoperative treatment was the same for both groups. Mobilization started on the first day after surgery with use of two forearm crutches with four-point walking. The use of two crutches was recommended for three weeks postoperatively. Patients were allowed to dispense with the crutches for full weight-bearing as soon as possible, depending on the individual level of mobilization and pain. Additional intensive physical therapy was started on the first day and was continued until the time of suture removal. All patients were discharged after a minimum hospital stay of ten days (range, ten to thirteen days).
Analysis and Statistical Evaluation
All forty patients were evaluated at three different time points: one day preoperatively, ten days postoperatively, and twelve weeks postoperatively (to cover the early postoperative period).
A three-dimensional gait analysis was conducted with use of a passive Cleveland Clinic marker set and six cameras (Motion Analysis, Santa Rosa, California). Data collection and calculation were performed with use of Eva 6 and Ortho Trak 6.33 software (Motion Analysis Corporation). After placement of the markers, the data were collected while the patient walked an 8 to 10-m distance at his or her own chosen speed without the use of crutches. The sampling rate was 60 Hz. A minimum of five walking trials were recorded for each patient at each testing time point, and the trials were averaged. Ground reaction forces were captured during the walking trials with use of two force plates on a strain gauge base located halfway onto the walkway. A minimum of five force plate hits with the left and the right foot were recorded. Filtering of these data was carried out with use of a second-order Butterworth filter at a frequency of 6 Hz. The parameters were extracted with MATLAB 7.0 software (The MathWorks, Natick, Massachusetts).
For the statistical analysis, the temporospatial variables of velocity, cadence, step length, and stride length were evaluated. The kinematic variables that were analyzed were the range of flexion and extension of the operatively treated hip, pelvic tilt, and the presence of a Trendelenburg or Duchenne limp. To measure the Trendelenburg gait, markers were placed on the anterior superior iliac spines and the base of the sacrum. A drop of the marker on the swing side in correlation with the neutral level was defined as a positive Trendelenburg gait.
Variable differences between the two groups measured at the three time points (preoperative to ten days, ten days to twelve weeks, and preoperative to twelve weeks) were tested for normal distribution with use of the Kolmogorov-Smirnov test. The Student t test was used for continuous values, and the chi-square test was used for dichotomous variables. The level of significance for between-group comparisons was set at p < 0.05.
In addition, for twenty-five patients, including twelve from the minimally invasive group and thirteen from the standard group, a complete surface electromyographical evaluation was possible. For thirteen patients, including seven from the minimally invasive group and six from the standard group, no electromyography could be performed due to technical problems. For the two other patients, including one from the minimally invasive group and one from the standard group, the recorded signals were insufficient and had to be excluded from our analysis (Fig. 1-B). Muscle activations were recorded for the gluteus medius, the gluteus maximus, the rectus femoris, the tensor fasciae latae, the adductor group, and the erector spinae with use of self-adhesive surface electrodes. To reduce the resistance of the skin, an abrasive preparation gel was used. The electrodes were placed longitudinally over the muscles. The myoelectric signals during the walking trials were amplified (Myosystem 2000; Noraxon, Scottsdale, Arizona) and were sampled at a frequency of 1000 Hz (Myoresearch 1.60 Master Edition; Noraxon). At least three trials were captured for further analysis.
The trial was registered in the ClinicalTrials.gov Protocol Registration System with the ClinicalTrials.gov Identifier NCT00831363.
Source of Funding
There was no external funding for this study.
Complications
There were no intraoperative complications or periprosthetic fractures. Two patients, including one in the minimally invasive group and one in the standard group, had an early dislocation of the hip at home (one after a fall and the other after rising from a low chair). Both patients had a closed reduction under general anesthesia. No postoperative limb-length discrepancy exceeded 1 cm. Malpositioning of the components was not found on the postoperative radiographs.
Temporospatial Variables
There were no significant differences between the minimally invasive group and the standard group with regard to the temporospatial parameters (velocity, cadence, step length, and stride length) at any time point tested. Both groups showed decreased values at the ten-day time point in comparison with the preoperative values, with an increase by twelve weeks after surgery. Ten days postoperatively, patients in the minimally invasive group demonstrated a greater reduction of step length. At the twelve-week time point, both groups showed equal values (Fig. 2).
Gait Kinematics
Range-of-motion variables were measured on the basis of the gait analysis, not on the basis of a physical examination. These measurements specify the movement of the hip during the gait cycle and are not a measurement of the maximum clinical hip flexion and extension. With regard to the mean preoperative range of motion, the standard group showed a slightly higher value by 0.7°, which was not significantly different from the value for the minimally invasive group. With regard to the absolute values, the range of motion was decreased at ten days postoperatively and then increased at the twelve-week time point in both groups. Comparison of the differences between the ranges of motion during the gait cycle tested preoperatively and at the ten-day time point revealed that the minimally invasive group had a minor decrease in range of motion that was not significant from the decrease in the standard group (6.4° compared with 7.4°; p = 0.7). Between the ten-day and twelve-week time points, the standard group had slightly more improvement than did the minimally invasive group (13.5° compared with 13.0°; p = 0.8). In order to further analyze the postoperative reduction in the total range of motion of the operatively treated hips in both groups, maximum hip flexion during the swing phase was evaluated. Between the preoperative and ten-day postoperative time points, both groups showed a slight decrease, with no significant difference between the groups (1.95° in the minimally invasive group and 2.0° in the standard group). Between the ten-day and twelve-week time points, the mean hip flexion in the standard group had an improvement of 2.2°, whereas the minimally invasive group had a further slight decrease of 2.2° (p = 0.08). Preoperatively, the patients in the standard group had better hip extension (by a mean of 2.9°) at the terminal standing phase. This difference was not significant. Both groups demonstrated reduced extension at ten days and regained extension at twelve weeks. Examination of the differences between the preoperative and ten-day time points revealed that the minimally invasive group had a smaller decrease in hip extension in comparison with the standard group (4.4° compared with 5.3°); however, this finding was not significant (p = 0.7). Between ten days and twelve weeks, the minimally invasive group demonstrated more improvement than did the standard group with regard to hip extension (15.4° compared with 11.4°; p = 0.1) (Fig. 3).
Ten days after surgery, fifteen patients in the minimally invasive group and sixteen patients in the standard group had an increased anterior pelvic tilt at the terminal standing phase (p = 0.7). Between the second and third time points, the minimally invasive group again showed better improvement in comparison with the standard group, but again the difference was not significant (p = 0.5) (see Appendix).
In the entire study group, there was only one patient (in the standard group) with a positive Trendelenburg gait pattern at the ten-day time point. This finding had disappeared completely by twelve weeks after surgery. A conventional Duchenne limp pattern—a lift of the pelvis on the swing side combined with a bent trunk to the stance side—was observed in four patients in each group at ten days after surgery. At the three-month time point, the Duchenne limp was still observed in two patients in the minimally invasive group and in three patients in the standard group (p = 0.6). In addition, ten patients (five in each group) presented with a modification of the classic Duchenne limp, lifting the pelvis on the side of the swinging limb and holding the trunk upright without bending.
Electromyographical Evaluation
With regard to the activity of the gluteal muscles, five patients in the minimally invasive group and six patients in the standard group demonstrated an alteration in the electromyographical patterns at the ten-day time point as compared with the preoperative time point. A prolongation of the activation of the gluteal muscles during the midstance period was observed in one patient in the minimally invasive group, compared with five patients in the standard group (p = 0.08). A reduced activity time was seen in four patients in the minimally invasive group, compared with one patient in the standard group (p = 0.1) (Fig. 4).
With regard to the electromyographical pattern of the tensor fasciae latae, no patient in the minimally invasive group showed changes in muscle activity in comparison with the preoperative values. In the standard group, prolonged firing of the tensor was observed in three patients and a reduction of firing was observed in two patients (p = 0.02). This difference between the two groups was still seen at the three-month time point (see Appendix).
The electromyographical evaluation of the erector spinae, the group of adductors, and the rectus femoris showed no significant difference between the two groups.
In the present study, hip extension was reduced in both groups, with no significant difference between the two groups at ten days after surgery. This deficit was also reported by Perron et al.13. In their three-dimensional gait analysis, the gait patterns of eighteen women with a total hip arthroplasty were compared with those of fifteen healthy women. The authors of that study suggested that a deficit in hip extension is not caused by the weakness of the hip extensors—it is in fact a result of contractures of the flexor muscles and the joint capsule. To compensate for the reduced hip extension in the terminal stance phase and to obtain step length, both groups had development of an increased anterior pelvic tilt in the sagittal plane. This finding is a typical compensatory mechanism and has been observed in other studies6,13,14.
The Trendelenburg limp may occur in patients after total hip arthroplasty and is a clinical sign of gluteal insufficiency. In the present study, only one patient demonstrated an isolated Trendelenburg gait pattern. One reason for this rare finding is that we did not evaluate the clinical static Trendelenburg sign, which is defined as a drop of the pelvis on the contralateral side while the individual stands on the operatively treated limb. The presented measurements are gait analysis values and describe the dynamic Trendelenburg pattern. During the gait cycle, additional mechanisms occurred in nearly half of all patients to compensate for an isolated drop of the pelvis. The main compensatory pattern was a Duchenne limp, which is described as bending of the trunk toward the stance side combined with an additional lift of the pelvis on the side of the swinging limb. This gait pattern was seen in 20.0% of the subjects in each group. Additionally, five patients in each group showed a modified Duchenne limp—a lift of the pelvis on the swing side combined with a straight spinal column. This gait pattern is a compensation to reduce the load of the hip joint15. The data may indicate that there was no gluteal dysfunction in both groups because of the different surgical approaches, and they confirm the findings of cadaver studies suggesting that muscle damage after a minimally invasive approach may be even greater in comparison with that after the use of standard techniques16. The impact of this muscle damage on early functional recovery and walking ability is unknown. In gait analysis studies in the literature, an isolated Trendelenburg pattern has been a rare finding because of several compensation mechanisms. Madsen et al.17 examined the effect of two different surgical approaches (anterolateral and posterolateral) on gait in ten patients each. In that study, a pelvic drop on the swing side was rarely seen in either group, with no difference between the groups. This finding was explained by the loss of elasticity due to scarring of the tissues on the lateral side of the hip, which can reduce the amount of the pelvic drop below the horizontal during the stance phase. Such a mechanism could mask the gluteal weakness. In addition, when the trunk is inclined over the operatively treated hip, the contralateral side of the pelvis can be elevated through the pull of the abdominal wall muscles. This mechanism is similar to what was seen in the present study (Duchenne pattern).
We found no significant differences between the minimally invasive and standard approaches with regard to the temporospatial parameters (velocity, cadence, step length, and stride length). These data support the results reported by Meneghini et al.18. In that randomized, prospective study, the gait parameters for twenty-three patients were compared between three different minimally invasive surgical approaches (two-incision, mini-posterior, and mini-anterolateral [described as a modified Hardinge approach, comparable with the approach in our study]). All groups demonstrated improvements in gait parameters at six weeks postoperatively, with no difference being identified between the three approaches. In the prospective, randomized study by Bennett et al.6, nine patients undergoing a minimally invasive replacement and eight patients undergoing a standard replacement were compared with use of gait analysis. In both groups, the same posterior approach with the same implant was used by a single surgeon; the only difference was the length of the incision. Similar to our results, the minimally invasive group showed no significant improvement in comparison with the standard incision group in terms of temporospatial variables. The limitation of that study was that no different functional outcome in gait analysis should have been expected as the surgical approaches only differed in terms of the length of the skin incision. In the study by Dorr et al.7, patients who were managed with a posterior mini-incision approach were compared with those who were managed with a standard posterior approach in terms of early pain relief and function after total hip arthroplasty. A complete gait analysis was performed for twenty-five patients preoperatively and at six weeks and three months postoperatively. No differences were found between the two groups. In a clinical study, DiGioia et al.19 noted a significant improvement with regard to limping and the ability to climb stairs in the mini-incision group at three months. The problem with that study was that, in both groups, a posterior approach with a split of the gluteus maximus and a release of the short external rotators was performed. Again, the only difference was the length of the skin incision. Limping was measured with use of the Harris hip score. Patients were assessed after three, six, and twelve months. Therefore, the early postoperative period was not covered. However, the main benefit of a minimally invasive technique is stated to be improved mobilization, especially during the weeks immediately after surgery4,6.
With regard to electromyographical pattern differences, about half of the patients in both of our study groups presented postoperatively with an alteration of activity in the abductor group. A prolonged gluteal muscle signal over the midstance period was observed more frequently in the standard group. A similar electromyographical pattern was described by Perron et al.13 as a supporting mechanism against collapse of the operatively treated hip while it was less extended. Another finding was the abnormal prolongation of the activity of the tensor fasciae latae, which occurred only in the standard group. These abnormal muscle activities may indicate that the abductor group and the tensor fasciae latae are more irritated by the use of a transgluteal approach that involves splitting the fascia lata and the distal portion of the gluteus medius than by the use of a minimally invasive approach20. Baker and Bitounis11, in an electromyographical study, showed that weakness of the abductor muscles after a lateral Hardinge approach to the hip is caused either by the detachment or avulsion of the gluteus medius from the greater trochanter or by a traction injury to the superior gluteal nerve, which results in denervation of the tensor fasciae latae. Nevertheless, in the present study, the electromyographical findings were not associated with any clinical effect and showed no signs of influencing gait in either group. This finding could be explained by the fact that we used a soft-tissue-preserving technique in the standard group, in which a gluteus medius split is performed only for 3 cm above the upper tip of the greater trochanter, thereby minimizing the risk of injury of the superior gluteal nerve. Kenny et al.21 found no correlation between signs of an injury of the superior gluteal nerve in the electromyographical data and a positive Trendelenburg sign. Abitbol et al.22 detected a subclinical gluteal nerve injury in 77% of patients with use of electromyography, irrespective of whether a posterior or a lateral approach was used. These findings suggest that factors other than injury to the superior gluteal nerve due to the approach may be responsible for clinical signs of abductor weakness. The cause of a Trendelenburg gait after a total hip arthroplasty seems to be multifactorial.
The limitations of the present study include the relatively small number of patients in each group, which limited our ability to obtain sufficient statistical power. Gait analysis including a dynamic electromyographical examination is a very time-consuming method and takes approximately two hours per patient at any time point tested. Therefore, similar to the present study, previous studies that have included three-dimensional gait analysis have included small numbers of patients6,7,10,17,18,23. Nevertheless, we believe that because of the strict patient-selection criteria, data are appropriately stratified and still have value. Age at the time of surgery, body mass index, and especially preoperative function are predictors of postoperative recovery. Our study groups did not show a significant difference with regard to the female:male ratio. However, the groups did not differ significantly in terms of preoperative gait parameters. For this reason, we assume that sex-related differences did not have an impact on our results as no differences were found between the groups in any gait parameters postoperatively. In addition, no different sex-specific functional outcomes after total hip arthroplasty have been reported in other studies24,25.
The main goal of the present study was to evaluate the early postoperative period. Therefore, a maximum duration of follow-up of only twelve weeks after surgery was defined to cover the critical period when the benefit of a minimally invasive total hip arthroplasty has been reported to be greatest in terms of mobilization and rehabilitation4,6,7,26. Another limitation is that neither the surgeon nor the staff treating the patients after the operation were blinded to the technique used. This could have created a bias due to the expectation that patients who had received a minimally invasive total hip arthroplasty would improve faster with regard to walking ability. With regard to generalizability, the fact that we have a special joint replacement unit may influence the reproducibility of our results. We do not know how reproducible our data will be without such a team. In addition, our surgical approach for standard total hip arthroplasty is a maximum-tissue-preserving technique and may not differ significantly from what other surgeons regard as a minimally invasive approach. This fact may be one reason why no major differences were found between the two groups. Several authors have discussed the variability and repeatability of gait analysis and electromyographical data. Kadaba et al.27 attributed errors in the application of the markers to be a major reason for variations in the gait analysis data. To minimize this source of error, the markers on all patients in the present study were applied by the same two technicians (A.K. and B.A.), who specialize in gait analysis. Another inconsistency can occur as a result of skin movements, especially in obese patients. To exclude this variability, only patients with a body mass index of <30 kg/m2 were included. Nevertheless, some inconsistency is inevitable, especially with the placement of the markers for the trunk and the thigh.
In conclusion, with use of the objective method of three-dimensional gait analysis, the present study could not confirm the expected functional benefit of a minimally invasive total hip arthroplasty technique without muscle detachment in the early postoperative period in comparison with a standard transgluteal approach. In terms of the improvements in gait parameters, only marginal differences were detected in favor of the minimally invasive technique.