Postural Balance During Quiet Standing in Patients With Total Hip Arthroplasty With Large Diameter Femoral Head and Surface Replacement Arthroplasty

Article Outline

• Abstract
• Methods
  • Study Group
  • Radiographic Analysis
  • Postural Tasks
  • Statistical Analysis
• Results
  • Radiographic Analysis
  • Postural Variables
  • Strength Variable
• Discussion
  • Postural Stability
  • Effect of Prosthesis Type
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Nantel J, Termoz N, Ganapathi M, Vendittoli P-A, Lavigne M, Prince F. Postural balance during quiet standing in patients with total hip arthroplasty with large diameter femoral head and surface replacement arthroplasty.

Objective

To compare postural balance between patients who have had either a large diameter head total hip arthroplasty or surface replacement arthroplasty.

Design

Observational study.

Setting

Outpatient biomechanical laboratory.

Participants

Two groups of 14 patients with surface replacement or large diameter head total hip arthroplasties recruited from a larger randomized study and 14 control subjects.

Interventions

Not applicable.

Main Outcome Measures

Postural balance during quiet standing in dual and one-leg stance (operated leg), hip abductor muscle strength, clinical outcomes, and radiographic analyses were compared between groups.

Results

Compared to the control group, patients in both groups showed smaller center of pressure displacement amplitude in the medial-lateral direction in dual stance. Patients with large diameter head total hip arthroplasty showed lower hip abductor muscle strength compared to control subjects. There was statistical difference between the 2 patient groups in biomechanical reconstruction of the hip. Despite these differences, there was no significant difference in the ability to complete the one-leg stance task between the 3 groups.

Conclusions

The muscular strength in the operated limb could be mainly responsible for the lower center of pressure displacement amplitude compared to control subjects. However, the ability to complete the one-leg stance demonstrates that patients do not fear to load the hip prosthesis when needed. The large diameter femoral head may be a major mechanical factor contributing to these results.

Key Words: Arthroplasty, Biomechanics, Hip, Posture, Rehabilitation

List of Abbreviations: COM, center of mass, COP, center of pressure, RMS, root-mean-square, SRA, surface hip arthroplasty, THA, total hip arthroplasty

TOTAL HIP ARTHROPLASTY is a common and successful procedure that relieves pain and improves hip function, with an overall high patient satisfaction.¹, ² Functional and clinical outcomes are recognized to be quite positive, with reported gains in hip isometric strength,³, ⁴ restoration of gait velocity,⁵, ⁶ and improved quality of life in the early period of postsurgical recovery.²

Despite these positive outcomes, residual functional disabilities persist after conventional THA using smaller diameter heads (22 to 32mm head) and affect major daily living activities such as postural balance. Previous studies have reported deficits in postural control 6 to 12 months after surgery.⁷, ⁸, ⁹ More specifically, previous studies reported a lower endurance on the operated limb compared to the contralateral limb during one-leg standing⁹ as well as a restricted amplitude in the trunk movement during quiet standing.⁷ A more commonly reported deficit is a general weakness in abductor muscles after THA.⁵, ¹⁰ This point has important functional implications in postural control because the hip abductor muscles are strongly implicated in the medial-lateral control of balance.¹¹

A recent study highlighted difficulties in patients after conventional THA to maintain one-leg stance posture and a tendency to avoid loading the prosthetic hip during the dual stance task.¹² However, these results suggested that although muscle strength is an important factor to fully regain normal postural balance, other factors, such as the characteristics of the prosthetic, could also be important. More specifically, the hypothesis was that a more anatomical bearing surface versus the smaller femoral heads used in THA (22 to 32mm) might greatly affect postural balance. Large diameter head components can be used in 2 types of prostheses: the THA with large diameter femoral heads, and the SRA of the hip. Both prostheses have specific characteristics that might also affect postural regulation processes. Among them, the presence or not of a metallic stem in the medullar canal could be an important factor.

SRA has been recognized to better restore normal hip anatomy.¹³, ¹⁴ It has been proposed that the preservation of the femoral head and neck in the SRA allows a more physiologic load transmission to the proximal femur as well as a better proprioception.¹⁴, ¹⁵, ¹⁶ Large diameter femoral head in SRA and large diameter head THA improve joint stability in both patients with recurrent dislocations and those undergoing revisions.¹⁷, ¹⁸, ¹⁹ However, until now, to our knowledge, no study has investigated the functional differences between large diameter head THA and SRA during a quiet standing task. This comparison is important to understand the relative importance of large diameter head and other prosthetic characteristics, such as preservation of the native femoral neck. Therefore, the aim of this study was to compare postural stability in patients after they underwent hip arthroplasty with either large diameter head THA or SRA.

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Methods 

Study Group 

Forty-two subjects divided in 3 groups (14 controls, 14 large diameter head THA, and 14 SRA) participated in the study. Patients were selected on the basis of unilateral hip disease classified by consensus of the 3 surgeons (P.A.V., M.L., and A.G.R.). Exclusion criteria for all subjects included the presence of any condition that may have affected balance (contralateral hip arthroplasty or osteoarthritis, knee osteoarthritis, knee arthroplasty, neurologic or musculoskeletal impairments, and reported falls for the past 6mo). After surgery, all patients underwent a 12-week rehabilitation program including isometric and stretching exercises that targeted the knee and hip flexor-extensor muscle groups as well as the hip adductor-abductor muscles and internal-external rotator muscles. They were evaluated at a short-term follow-up (range, 5–7mo). The healthy control subjects were age-matched volunteers recruited from the community through our institutions. The descriptive characteristics of the groups are presented in table 1. All participants provided their written consent, and the project was approved by our institutional review boards.

Table 1. Characteristics of the 3 Groups

NOTE. Values are mean ± SD or as otherwise noted.

Abbreviations: BMI, body mass index; F, female; LDH, large femoral head diameter; M, male.

Three experienced surgeons performed the surgeries. In all cases, they used a posterior surgical approach. In the SRA group, the hybrid Durom hip-resurfacing system^a was implanted (fig 1). In the large diameter head THA group, a CLS Spotorno titanium uncemented femoral stem^a was used with a large diameter modular metal head (Metasul) and a Durom acetabular component^a (fig 2). Figure 3 shows a radiograph of the pelvis of a patient (not included in the study) with both types of prostheses. During each procedure, the surgeons tried to reproduce the patient's hip anatomy by using preoperative templating with the opposite side as a reference and by using bony landmarks. Surgical techniques for all the procedures as well as the radiographic analyses have been described previously.¹³, ²⁰, ²¹

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• Fig 1. 

  The hybrid Durom hip surface replacement arthroplasty system with chrome-cobalt femoral head and acetabular cup.^a

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• Fig 2. 

  The CLS femoral stem, the Metasul large diameter head, and the Durom acetabular component.^a

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• Fig 3. 

  Radiograph example of the pelvis of a subject (not included in the study) with a large diameter head THA on the right hip and an SRA on the left hip.

Radiographic Analysis 

Standardized postoperative radiographs were analyzed. Anterior-posterior radiographs of the pelvis were taken with the legs positioned in 15° of internal rotation. The radiographs were rejected if the coccyx was not centered on the pubic symphysis and located proximally within 2 to 4cm. This ensured proper positioning of the pelvis in both frontal and sagittal planes. These were scanned (VIDAR VXR-12^b) and analyzed by Imagika software.^c The femoral offset, the horizontal and vertical centers of rotation, and leg-length inequality were measured for the replaced and normal contralateral hip on postoperative radiographs. The femoral offset was defined as the perpendicular distance (mm) from the center of rotation to the femoral shaft line. The vertical hip center of rotation was determined by the perpendicular distance (mm) from the center of rotation of the hip to the interteardrop line. The horizontal center of rotation was evaluated by the distance (mm) between the vertical center of rotation line and the teardrop. Finally, the leg-length inequality was evaluated by the perpendicular distance (mm) from the superior aspect of the lesser trochanter to the interteardrop line.

Postural Tasks 

Each participant had to achieve 2 postural tasks. The first task consisted of quiet standing for 120 seconds with eyes open. In the second task, patients had to hold a one-leg stance position on the operated limb for 10 seconds. The operated leg was tested twice with a sufficient intertrial resting period. This task was considered successful when the patient was able to stay still on 1 leg for 10 seconds and was considered unsuccessful if the patient had to touch the ground with the contralateral foot. The abductor muscles strength of both legs was assessed with a handheld dynamometer.^d All the subjects were tested in the sitting position with hip and knee flexed at 90°. The dynamometer was placed proximal to the lateral femoral epicondyle, and subjects were asked to perform maximal isometric contraction. Testing was done 3 times.

To limit the interexaminer variability, the peak force value generated by the abductor muscles of the operated limb (in control subjects, the left limb was used for this measurement) was expressed as the percentage of the peak force generated by the abductor muscles of the sound limb (in control subjects, the right limb was used for this measurement). For the dual-leg stance task, ground reaction forces and moments were collected with an AMTI forceplate^e recording at a sampling frequency of 60Hz. The signal was low-pass filtered at 2Hz with a dual-pass second-order Butterworth digital filter. The time histories of the COP profiles were calculated from the orthogonal forces and moments recorded by the forceplate, and the COM displacement was estimated from the zero-point to zero-point integration technique.²², ²³

The average COP position in the medial-lateral direction was calculated over the 120 seconds of the trial and expressed from the fore-aft midline of the base of support delimited by reflective markers on heels and toes. Then the COP position within the base of support was graphically analyzed by Matlab 5.1.^f The load was considered symmetric if the average position of the COP was in the middle of base of support. Therefore, a shift from this position was considered asymmetric and interpreted as the patient's preferred loading leg. The COP and COM displacement profiles were reported as the RMS amplitude (in mm) of COP and COM in both the anterior-posterior and medial-lateral directions.

Statistical Analysis 

For dual stance and abductor muscles relative strength, we compared the differences between groups with 1-way analyses of variance. For the dual stance task, we used the RMS amplitude of COP and COM in the anterior-posterior and in the medial-lateral directions and the abductor muscles strength percentage as dependent variables. The statistical significance was set at P less than .05, and Newman-Keul post hoc analyses were conducted when necessary. To evaluate radiographic mean differences between the large diameter head THA and SRA groups, we used Student t tests (P<.05). Finally, we used a Pearson chi-square test to test dependence between group and one-leg stance task success.

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Results 

As shown in table 1, we found no significant differences in the groups' characteristics.

Radiographic Analysis 

We observed differences between the large diameter head THA and SRA groups in the biomechanical reconstruction. As shown in table 2, the femoral offset differential between the operated limb and the sound limb was significantly different between the groups (P<.01). The femoral offset in the large diameter head THA group increased on the operated side and reached 106.9%±11.0% relative to the contralateral limb compared to the SRA group that showed a negative offset differential (P<.01) and consequently a relative offset that reached 94.3%±8.8% of the contralateral limb. We found no statistical differences for the horizontal (P=.95) and vertical center of rotations (P=.46) and for leg length inequality (P=.59). In both groups, the horizontal center of rotation was medialized while the vertical center of rotation was more proximal. The leg length of the operated leg relative to the contralateral leg was decreased in both large diameter head THA and SRA.

Table 2. Biomechanical Restoration of the Operated Hip Compared to the Sound Limb in Both THA With Large Femoral Head Diameter and SRA Groups

NOTE. Values are mean ± SD or as otherwise noted.

Abbreviation: LDH, large femoral head diameter.

Postural Variables 

The statistical analyses for the dual stance task revealed significantly lower COP RMS amplitudes in the medial-lateral direction for large diameter head THA and SRA subjects compared to control subjects (P=.04) (table 3). In this direction, the mean COP position was shifted on the sound limb in 12 of the 14 patients for both the SRA and large diameter head THA. The COM RMS amplitude values for both patient groups were smaller compared to those of the control group, but not significant (P=.09). We observed no significant differences between groups in the anterior-posterior direction for both RMS_COP (P=.58) and RMS_COM (P=.59) amplitudes. Although all the patients in the large diameter head THA group and control subjects were able to successfully complete one-leg stance, 1 patient in the SRA group was not able to complete it. Lack of balance was mentioned by this patient to explain his inability to maintain the position. The patient reported no pain. Statistical analysis showed no significant dependence between groups and one-leg stance task success (P=.36).

Table 3. RMS of the COP, COM Amplitudes (mm) in Medial-Lateral and Anterior-Posterior Directions and Abductor Muscles Strength in Control, THA With Large Femoral Head Diameter, and SRA Groups

NOTE. Values are mean ± SD.

Abbreviations: AP, anterior-posterior; LDH, large femoral head diameter; ML, medial-lateral; RMS_COP, RMS amplitude of COP; RMS_COM, RMS amplitude of COM.

Strength Variable 

Statistical analyses revealed a significant group effect in the abductor muscles strength of the prosthetic hip relative to the sound hip (P=.05). The post hoc analysis showed statistically lower strength for the large diameter head THA group compared to control (P=.04), whereas the SRA group showed lower but not statistically significant difference (P=.07). We found no difference between the patient groups (P=.47).

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Discussion 

The purpose of the present study was to compare postural stability in patients 6 months after they underwent hip arthroplasty either large diameter head THA or SRA to discriminate the effect of specific prostheses characteristics on postural control. Because both large diameter head THA and SRA have similar large diameter bearing surfaces, the influence of the size of the femoral head on postural control can be excluded.

Postural Stability 

Both the large diameter head THA and SRA groups showed smaller COP RMS displacement amplitudes than control subjects. This strategy to reduce the COP displacement amplitude has previously been reported in an elderly population during a prolonged dual stance task.²⁴ The authors suggested that a deficit in the load-unload mechanism could have been responsible for that strategy. In our groups, the limited displacements could be a strategy to reduce the contribution of the hip abductor muscles on the prosthetic side. Because patients showed lower hip abductor muscles strength, they might choose to reduce the contribution of the abductor muscle on the impaired hip. This hypothesis is strengthened by an asymmetric loading, as demonstrated by the mean COP position on the sound limb in 12 of the 14 patients in large diameter head THA as well as SRA groups.

However, considering the important contribution of the hip abductor muscles to control medial-lateral postural balance during the one-leg stance, it is surprising that only 1 of the 28 arthroplasty patients failed to complete the task. This result is different from previous studies that reported instability or failure to complete a one-leg stance after conventional THA.⁹, ¹² This result suggests that compared to patients with conventional THA, large diameter head THA and SRA patients are confident in their prosthesis, and they do not fear to load it for a short period (10s). A recent study reported lower postural control in patients with conventional THA compared to both SRA and control group.¹² This result was attributed to 3 possible factors: femoral stem in THA versus none in SRA, 28mm head diameter in THA versus larger anatomical head diameter in SRA, and the difference in the biomechanical reconstruction achieved by the 2 procedures (reduced femoral offset in SRA vs increased in THA).

Effect of Prosthesis Type 

Because both large diameter head THA and SRA use anatomical diameter femoral head, the main difference between the 2 prostheses is the presence (large diameter head THA) or absence (SRA) of stem in the medullar canal. In the SRA group, part of the femoral head and neck is preserved, which may make it easier for the surgeon to restore normal hip joint biomechanics. On the other hand, the femoral stem in the THA imposes bone remodeling that can lead to nonoptimal hip joint biomechanics. It has also been proposed that the conservation of the proximal femur in SRA may result in a more physiologic loading¹⁴, ¹⁵, ¹⁶ and avoids the development of thigh pain or discomfort.²⁵ Nevertheless, in the current study, we observed no difference in postural control between the 2 groups of patients. Consequently, neither the femoral stem in the large diameter head THA nor the more physiologic loading in the SRA group seems to affect the postural control, or at least its effect may be attenuated by another factor.

Previous studies reported an increase in joint stability and a decrease in impingement and dislocation rate in patients with large diameter femoral head.²⁶, ²⁷ Some authors also proposed that the large femoral head could possibly allow a more anatomical hip joint fitting compared to smaller femoral head.¹³, ¹⁴ In the current study, the anatomical fitting of the large diameter femoral head component could have played an important role in the patients' ability to load the prosthetic side during the one-leg stance. It is therefore possible that the large head component has helped large diameter head THA and SRA patients to maintain postural stability in comparison to patients with conventional THA.⁷, ⁸, ⁹, ¹²

However, to assure maximal stability in the prosthetic joint, the large femoral head component must be associated with optimized hip anatomy restoration.²⁶ It has been reported that SRA allows a more anatomical reconstruction of the hip anatomy than conventional THA.¹², ¹³, ¹⁴ The radiographic analysis presented here showed that unlike conventional THA, large diameter head THA allowed biomechanical reconstruction comparable to SRA with regards to restoration of vertical and horizontal center of rotations and for the leg length equalization. With the improved intraoperative joint stability provided by anatomical head size components, the surgeons did not have to increase leg length to compensate unsatisfactory stability, as it is common with the 28mm head diameter. In the SRA group, the femoral offset was decreased compared to the contralateral hip joint (average ± SD, −2.49±3.67mm) while it was increased in the large diameter head THA (average ± SD, 2.97±5.39mm). This is consistent with previous studies on femoral offset reconstruction in THA¹⁰ and SRA.¹³ However, the difference in the femoral offset restoration did not seem to play a significant role in affecting postural stability in the current study. Therefore, out of the 3 factors, the large diameter femoral head could be the main factor explaining the difference between postural control in our SRA and large diameter head THA groups and results reported in previous studies in conventional THA.⁷, ⁸, ⁹, ¹²

Nevertheless, when considering the results in hip abductor muscle strength, it seems that even with optimum conditions, including joint reconstruction, appropriate prosthesis, and use of a posterior approach that does not violate the hip abductors, patients do not recover full abductor muscle function even at 6 months after surgery. The abductor weakness may also have been long-standing as a result of pain and dysfunction resulting from the arthritic process itself. However, clinical scores after THA can improve up to 1 to 2 years after surgery,², ²⁸ so further evaluation is needed to determine whether full abductor strength will be recovered. As proposed by Trudelle-Jackson and Smith,⁹ a home-based program targeting specifically abductor muscles and weight-bearing exercises must be initiated after the standard early postsurgery rehabilitation program. Indeed, training programs with major emphasis on muscle strengthening and weight-bearing seem to provide the best results in terms of improvement in functional level, postural balance, and mobility after THA.⁹, ²⁹ Furthermore, it has been suggested that the length of the rehabilitation program should be increased⁴, ⁹, ²⁸, ²⁹, ³⁰ to assure the complete regain of the hip abductor strength. For the patients, this could mean a faster return to their normal daily living activities as well as prevent falls and injuries during more challenging activities such as physical activities and sports.

Study Limitations 

To determine the time it takes patients to return to normal postural control and to fully regain muscular strength, it would be important to repeat this study with a longer follow-up. The generalization of the results could also benefit from a larger sample size of patients.

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Conclusions 

The present study showed that large diameter head THA and SRA patients did not fully recover abductor muscle strength 6 months after surgery. This lower abductor muscular strength could be mainly responsible for the lower COP displacement amplitude in the medial-lateral direction compared to the control. This strategy could be done to minimize the contribution of the hip abductor muscles on the prosthetic side and avoid postural instability during a long-duration task. However, the achievement of the 10-second one-leg stance task demonstrates the patients' ability to load the prosthetic hip with confidence when necessary. The results suggest that in spite of differences in prostheses characteristics and joint reconstruction between large diameter head THA and SRA, the large femoral head component seems to be the critical mechanical factor leading to postural stability.

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