Gait Patterns After Total Hip Arthroplasty and Surface Replacement Arthroplasty

Article Outline

• Abstract
• Methods
  • Participants
  • Clinical Outcomes, Intervention, and Radiographic Analysis
  • Postoperative Rehabilitation Intervention
  • Gait-Assessment Procedures
  • Instrumentation
  • Kinematic and Kinetic Analyses
  • Statistical Analysis
• Results
  • Group Characteristics
  • Clinical Outcomes and Radiographic Analysis
  • Strength Variable
  • Gait Parameters
    • Spatiotemporal
  • Mechanical Work
    • Sagittal plane
  • Frontal Plane
• Discussion
  • Mechanical Work in the Sagittal Plane
  • Mechanical Work in the Frontal Plane
  • Effect of Biomechanical Reconstruction
  • Musculoskeletal Rehabilitation Considerations
• Conclusions
• Acknowledgment
• References
• Copyright

Abstract 

Nantel J, Termoz N, Vendittoli P-A, Lavigne M, Prince F. Gait patterns after total hip arthroplasty and surface replacement arthroplasty.

Objective

To compare gait patterns in patients with total hip arthroplasty (THA) and surface hip arthroplasty.

Design

Observational study.

Setting

Outpatient biomechanical laboratory.

Participants

Two groups of 10 surface hip arthroplasty and THA patients and 10 control subjects participated in the study (N=30). The patients were volunteers recruited from a larger randomized study.

Interventions

Not applicable.

Main Outcomes Measures

Gait patterns, hip abductor muscle strength, clinical outcomes, and radiographic analyses were compared between groups.

Results

In the sagittal plane, the THA group showed a larger flexor moment and larger mechanical work in H2S and K3S power bursts compared with surface hip arthroplasty and control subjects. In the frontal plane, both THA and surface hip arthroplasty patients had smaller hip abductor muscles energy generation (H3F) than the control group. No difference was found for the hip abductor muscles strength.

Conclusions

In the THA group, the larger energy absorption in H2S and K3S would be a cost-effective mechanical adaptation to increase stability. The surface hip arthroplasty characteristics could allow the return to a more normative gait pattern compared with THA. The modification in the frontal plane in surface hip arthroplasty and THA would be related to the hip abductor muscles strength.

Key Words: Arthroplasty, Biomechanics, Gait, Hip, Kinetics, Rehabilitation

Abbreviations: ANCOVA, analysis of covariance, ANOVA, analysis of variance, COM, center of mass, ROM, range of motion, THA, total hip arthroplasty, WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index

THE DEMOGRAPHIC CHARACTERISTICS of patients who underwent hip arthroplasty have changed over a few decades.¹ They are younger and more active.¹ Thus, these patients are considered as a challenge for the durability and the longevity of the prosthesis.¹, ², ³ To ensure high-standard clinical outcomes, extensive researches are performed regarding surgical approaches,⁴, ⁵ bearing surfaces,⁶, ⁷ and implant designs.¹

With long-term experience, conventional THA has proven its suitability and still provides high satisfaction in both patients and clinicians. However, because of the risks of dislocation with young or active patients, some authors proposed alternatives to THA. Among them, the surface replacement arthroplasty seems to be particularly adapted to this population.² Indeed, the absence of the stem in the medullar canal and the conservation of the femoral head appear as the main advantages of the surface hip arthroplasty over THA. This results in bone conservation that could facilitate the revision of the prosthesis or the conversion to a conventional THA prosthesis.², ⁸, ⁹ This has been pointed out as a crucial benefit in young or active patients who are more likely to need prosthesis revision over their lifetime.², ¹⁰, ¹¹

In a functional point of view, the postsurgical progresses observed after THA are known to be rapid because most of them take place between the first 3 and 6 months after surgery.¹² Unfortunately, some disabilities persist 1 to 2 years after surgery¹³, ¹⁴, ¹⁵ and affect important daily living activities, such as gait. Although fast improvements in gait spatiotemporal parameters have been reported, they do not reach normative values.¹⁴, ¹⁶, ¹⁷ These deficits might be attributed to abductor muscles impairments that are also associated with trunk compensation¹⁶ and an abnormal gait pattern.¹⁶, ¹⁷

The characteristics of the prosthesis itself could also be an important concern. Indeed, because of reduced bone resection, the surface hip arthroplasty may allow the anatomic preservation of the hip biomechanics (mainly the femoral offset, center of rotation position, abductor muscle lever arm)¹¹, ¹⁸, ¹⁹ that may lead to a more physiologic load transfer with respect to THA. In addition, their larger femoral heads have been recognized to contribute to the reduction of stress on the proximal femur.¹⁰, ¹¹, ¹⁹, ²⁰ It was reported that the surface hip arthroplasty's characteristics could be important factors allowing the return to normal postural stability during a quiet standing task.²¹ The assessment of the individual impact of surface hip arthroplasty and THA procedures on gait is of prime importance to highlight the best reconstruction method for gait recovery after hip arthroplasty. Recently, Mont et al¹⁷ reported a slower walking speed and a reduced hip abductor moment of force in THA patients and patients with osteoarthritis compared with surface hip arthroplasty patients 6 to 18 months postsurgery. These authors reported closer to normal gait parameters in surface hip arthroplasty but did not take the walking speed into consideration despite its effect on the gait pattern.¹⁶ Furthermore, a more restrictive follow-up may help optimize a comparison between patients. Therefore, the aim of the present study was to compare the effects of both prostheses on gait pattern 6 to 8 months postsurgery.

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Methods 

Participants 

Thirty subjects, divided into 3 age-matched groups (10 healthy controls, 10 THA, 10 surface hip arthroplasty), participated in the study. Among volunteers recruited from a larger randomized study,¹⁸, ²², ²³ patients were selected on the basis of unilateral hip disease without other problems that might interfere with gait. The patient's hip osteoarthritis diagnosis was classified with consensus by 3 surgeons on the basis of the clinical history, available laboratory tests, and radiographs. Exclusion criteria included contralateral hip arthroplasty or osteoarthritis, knee osteoarthritis, knee arthroplasty, morbid obesity, joint deformity, and back pain. They were evaluated at a short-term follow-up of 6 months (range, 6–8mo). The healthy control subjects were volunteers recruited from the community through our institutions and were age matched with patients. The control subjects were excluded if they reported musculoskeletal impairments (eg, lower-extremity joint replacement, diagnosed osteoarthritis, hip and knee pain) or any neurologic disorders. The descriptive characteristics of the groups are presented in table 1. All participants gave their written consent, and the project was approved by local research ethics and scientific committees.

Table 1. Descriptive Characteristics of Controls, Conventional THA, and Surface Replacement Arthroplasty Subjects

NOTE. Values are mean ± SD unless otherwise noted.

Abbreviations: NA, not applicable; SRA, surface hip arthroplasty.

Clinical Outcomes, Intervention, and Radiographic Analysis 

Clinical function was evaluated with the WOMAC and the Merle d'Aubigné-Postel preoperatively and at 6 months postsurgery. The interventions were performed by 3 experienced surgeons by using a posterior surgical approach. In the surface hip arthroplasty group, the Durom hip-resurfacing system^a was implanted. For the THA group, a CLS Spotorno^a titanium uncemented femoral stem^a was used with a 28-mm Metasul femoral head^a articulated with a Metasul bearing insert fitted into an Allofit uncemented acetabular cup.^a The surgical technique for all procedures and radiographic analysis have been described in previous studies.¹⁸, ²², ²³ The femoral offset (perpendicular distance in mm from the center of rotation to the femoral shaft line), the horizontal centers of rotation (distance in mm between the vertical center of rotation line and the teardrop), the vertical centers of rotation (perpendicular distance in mm from the center of rotation of the hip to the interteardrop line), and leg-length discrepancy (perpendicular distance in mm from the teardrop to the lesser trochanter's line) were measured for the replaced and normal contralateral hip on postoperative radiographs by using Imagika software.^b,18

Postoperative Rehabilitation Intervention 

After the acute medical supervision, all patients benefited from a rehabilitation program. The home-based program was realized on a daily basis. A first meeting with the physical therapist was set to explain and to modify the program according to the patients' physical condition. As an outpatient, a second meeting with the physical therapist was required to supervise the program execution.

The purpose of the program was to allow a rapid regain of joint ROM and muscle strength. The program included isometric and stretching exercises and targeted the knee and hip flexor and extensor muscle groups as well as the hip adductor-abductor muscles and internal-external rotator muscles. During the 12 weeks of the program, the progression was ensured by increasing the level of difficulty of the exercises. Indeed, for the first 4 to 5 weeks, most of the exercises were performed in a seated position with a small ROM. Technical aids were required for exercises performed in an upright standing position. At 6 to 8 weeks, patients were asked to increase the ROM during the stretching or functional exercises. Most of the exercises were performed in an upright standing position without an assistant device.

In the first postoperative weeks, activities such as walking with an assistance device, stationary cycling, and swimming were encouraged. Participation in high-impact physical activities (basketball, football, hockey, etc) was proscribed in the first 3 months and subject to restriction after this period.

Gait-Assessment Procedures 

Subjects were requested to walk at their normal cadence on a 10-m walkway. The starting point was selected to ensure steady-state gait before reaching the force platforms. Four gait cycles were collected for each subject. Trials were saved when both feet made full contact with each of the 2 force platforms. A sufficient resting period was given between trials to avoid fatigue. Subjects were tested barefoot and wearing tight fitting shorts and a t-shirt.

Instrumentation 

Nineteen 14-mm diameter reflective markers were used to define lower-limb body segments. The kinematic and kinetic data were collected at 60Hz by using 8 optoelectronic cameras^c and at 120Hz with 2 embedded force platforms,^d respectively. The abductor muscles' strength on both sides was assessed by using a handheld myometer.^e To limit the interexaminer variability, the peak force value (N) generated by the abductor muscles of the operated limb was expressed relative to the sound limb (%).

Kinematic and Kinetic Analyses 

Kinematic and kinetic parameters were derived by using VICON Clinical Manager.^c Cadence, duration of single- and double-support phases, stride length, and walking speed were calculated and analyzed for each trial. The mechanical power phases and moment of force peak values, the mean moment of force, and mechanical work were analyzed. Mechanical powers were calculated from the dot product of the joint angular velocity and the net joint moment. Mechanical work was calculated by the time integration of the power phases with respect to time.²⁴ In the sagittal plane, the mechanical power and work were calculated for the ankle, knee, and hip joints and at the hip in the frontal plane.

Statistical Analysis 

One-way ANOVAs were performed to compare the differences between groups for the relative strength of the abductor muscles and kinematic and kinetic parameters. As previously reported, speed affects the gait pattern. Therefore, we controlled for this parameter by using a 1-way ANCOVA. However, because of the nonsignificant difference in gait speed between the groups, the results from the ANCOVA did not differ from those of the ANOVA. Consequently, only the ANOVAs will be presented. We used paired Student t tests to assess the groups' within difference for clinical outcomes. We assessed within-group differences for postsurgical conditions and radiographic analyses with unpaired Student t tests. The statistical significance level was set at P less than .05, and Newman-Keuls post hoc analyses were conducted when necessary.

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Results 

Group Characteristics 

As shown in table 1, no significant differences were found between the groups for age, mass, height, and body mass index.

Clinical Outcomes and Radiographic Analysis 

Significant statistical differences were found in both groups for pre- and postsurgical conditions in the WOMAC and Merle d'Aubigné-Postel scales (P<.001). For the WOMAC, THA and surface hip arthroplasty showed respectively larger scores in the presurgical condition (mean ± SD, 52±17, 45±11) compared with the postsurgical condition (mean ± SD, 15±15, 7±4). In the Merle d'Aubigné-Postel, the THA and surface hip arthroplasty groups showed lower scores in the presurgical condition (mean ± SD, 12±3; 11±2) compared with the postsurgical condition (mean ± SD, 17±1; 18±0). Neither of the pre- or postsurgical conditions revealed statistical differences between groups in the WOMAC (P=.31 and P=.11) or Merle d'Aubigné-Postel scores (P=.82 and P=.22).

Differences in the hip joint reconstruction were observed between the groups (table 2). Both the femoral offset differential (in mm and %) between the operated and the sound limb were significantly different between the groups. The femoral offset in the THA group was increased on the operated side, whereas it was decreased in the surface hip arthroplasty group. Similarly, despite no significant differences, the length of the operated leg was increased in the THA group but decreased in the surface hip arthroplasty group. The THA and surface hip arthroplasty leg-length discrepancies between the operated and sound leg are concordant with a previous study¹⁸ and are mostly caused by hip joint reconstruction and prosthetic components. In both groups, the horizontal center of rotation was medialized, whereas the vertical center of rotation was more proximal.

Table 2. Hip Joint Reconstruction of the Operated Hip Compared With the Sound Limb in the THA and Surface Replacement Arthroplasty Groups

NOTE. Values are mean ± SD unless otherwise noted. P<.05.

Abbreviation: SRA, surface hip arthroplasty.

Strength Variable 

Statistical analyses revealed lower but not significant (P=.09) abductor muscle strength for the surface hip arthroplasty (mean ± SD, 91%±12%) and THA (mean ± SD, 91%±12%) subjects compared with the control subjects (left limb relative to right limb, mean ± SD, 104%±18%).

Gait Parameters 

The statistical analyses of the spatiotemporal parameters (table 3) revealed no significant differences between the groups for walking speed, stride length, and single- and double-support phases.

Table 3. Spatiotemporal Parameters Between the Controls, THA, and Surface Replacement Arthroplasty Groups

NOTE. Values are mean ± SD.

Abbreviation: SRA, surface hip arthroplasty.

Mechanical Work 

No significant differences were found in the mechanical work for the A1S and A2S power phases (ankle) or for the K1S, K2S, K4S (knee), and H3S power phases (hip) (table 4). Statistical difference was found between groups for the mechanical work during the K3S power burst (P=.05). The post hoc tests revealed that the K3S values were larger in the THA compared with the control (P=.04) and surface hip arthroplasty (P=.10) groups. The mechanical work for the H2S phase was also statistically different between groups (P=.02) with larger values in THA compared with the control (P=.03) and surface hip arthroplasty (P=.03) groups.

Table 4. Mechanical Work (J/kg) in the Sagittal and Frontal Planes at the Ankle, Knee, and Hip Joints for the Controls, THA, and Surface Hip Arthroplasty Groups

NOTE. Values are mean ± SD unless otherwise noted.

Abbreviation: SRA, surface hip arthroplasty.

Concerning the mean hip flexor moment during the H2S power phase, the statistical analysis revealed a main group effect (P=.02) with larger values in the THA group (–.55±.18Nm/kg) compared with the control (–.30±.10Nm/kg, P=.02) and surface hip arthroplasty (–.33±.10Nm/kg, P=.07) groups. The peak hip moment corresponding to the H2 power phase was also significantly different between groups (P=.03), with larger values in the THA group (–1.19±0.20Nm/kg) compared with both the control (–.84±.19Nm/kg, P=.05) and surface hip arthroplasty (–.72±.23Nm/kg, P=.04) groups (fig 1).

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• Fig 1. 

  Hip moment and power curves in the sagittal and frontal planes for control, surface hip arthroplasty, and THA groups. The dash-dot lines delimit the phases corresponding to the H2S power burst in the sagittal plane and the H3F in the frontal plane. A positive power value indicates an energy generation (concentric contraction), and a negative power value specifies energy absorption (eccentric contraction). The kinetic data were normalized to body weight and reported in percentage of the gait cycle to facilitate the intersubject comparison. The gait cycle was defined from 0% (first heel contact) to 100% (the subsequent ipsilateral heel contact), with the toe-off occurring at 60%. *THA is significantly different from the control and surface hip arthroplasty. †THA and surface hip arthroplasty groups are significantly different from the control group.

Frontal Plane 

In the frontal plane, no significant differences were found between groups for the mechanical work during the H1F and H2F power phases. However, the statistical analysis reported a significant main group effect (P<.01) for the mechanical work corresponding to the H3F phase with lower abductor muscle generation in the THA and surface hip arthroplasty groups compared with the control group (P<.01 and P<.01, respectively).

For the moment of force occurring during the midstance (10%–30% of the gait cycle), the surface hip arthroplasty showed a lower but not significant (P=.25) abductor peak value (.75±.13Nm/kg) compared with both the THA (.81±.18Nm/kg) and control subjects (.89±.23Nm/kg). During terminal stance and preswing (30%–60% of the gait cycle), the THA and the surface hip arthroplasty showed a lower but not significant (P=.27) abductor moment peak value (.65±.23Nm/kg and .72±.20Nm/kg, respectively) compared with the control group (.83±.21Nm/kg).

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Discussion 

The main objective of the present study was to compare gait patterns in patients with total hip arthroplasty and surface replacement arthroplasty. The statistical analysis showed differences between the groups in the sagittal plane, whereas both patient groups showed hip abductor muscle weakness that could be mainly responsible for the altered gait pattern in the frontal plane.

Hip arthroplasty is known to be an efficient way to relieve pain and improve functional capacity.¹², ²⁵ However, previous gait studies¹⁶, ¹⁷, ²⁶, ²⁷ reported a significantly lower walking speed in THA patients compared with healthy subjects 6 months to 2 years postsurgery. Based on their walking speed and other spatiotemporal parameters, patients involved here seemed to have an effective functional recuperation because they reached normal values within 6 months postsurgery. However, to obtain precise indication about the nature of the gait patterns, an analysis of mechanical work must be performed because this parameter presents advantages over moment of force peak values previously reported to compare THA and surface hip arthroplasty.¹⁷ Indeed, it highlights the functional role of the muscles and the potential mechanical adaptations.²⁴ Furthermore, mechanical work takes into account the time factor to realize the task.²⁴, ²⁶

Mechanical Work in the Sagittal Plane 

In the sagittal plane, the surface hip arthroplasty group mostly returns to a normative gait pattern, whereas the THA group does not. The main difference between groups has been observed at the hip, with THA absorbing more energy in hip flexor muscles (H2S) than both the surface hip arthroplasty and control groups. This larger energy absorption could be caused by the passive resistance of the hip flexor muscles. Indeed, Perron et al¹⁶ reported a reduction in the extension excursion at the hip joint during the early push-off phase. These authors proposed that hip flexor contracture could be responsible for it but could not confirm this hypothesis because passive ROM was not tested. Although this explanation is still possible, nothing allows supposing that THA patients could be more predisposed to develop hip flexor contracture compared with surface hip arthroplasty patients. However, this larger contribution of the hip flexor muscles has been previously reported as a neuromuscular adaptive mechanism to ensure forward progression in obese children²⁸ and disabled elders.²⁹ According to previous studies,¹⁶, ²⁶ THA showed a major deficiency in generating energy with the hip flexors (H3S). Our results do not confirm this result, but the large amount of energy absorbed by the hip flexor muscles during the H2S power phase could be, in part, used to enhance the hip mechanical work for the pull-off (H3S).²⁸, ²⁹ This hypothesis is also supported by THA patients absorbing a larger amount of energy with the knee extensors (K3S: 50%–60% gait cycle). With biarticular knee extensors also being hip flexors, the increased amount of energy absorbed by the THA in the K3S and in the H2S power phases could be used to optimize the generation of energy for the swing of the leg (H3S: ±50%–75% gait cycle).

Furthermore, the hip flexor/extensor muscles share the major role of controlling the large trunk's inertia in the plane of progression.³⁰ Consequently, THA subjects may use the energy absorption by the flexor muscles to enhance the control of the whole-body COM and then to ensure safe forward progression until the subsequent double-stance phase.³⁰ Consequently, the larger energy absorption during both the H2S and K3S phases could be used by the THA group as a cost-effective mechanical adaptation to enhance the stability of the COM and to allow an efficient forward progression without increasing the energy generation. However, these results highlight only a portion of the mechanical energy used to realize the task because the calculations do not take into consideration the possible mechanical energy generated by the antagonist muscle cocontraction.

Mechanical Work in the Frontal Plane 

In the frontal plane, both the THA and surface hip arthroplasty groups showed lower abductor energy generation (H3F) compared with controls. The abductor moment of force corresponding to the H3F power phase was also lower but not significant. Previous studies reported a lower peak abductor moment in the THA compared with controls¹⁶, ¹⁷ and proposed that abductor muscle strength could be responsible for this result.¹⁶ The 10% lower abductor muscle strength (operated relative to sound limb) in our patient groups could have challenged the raise of the pelvis (H3F). Dissimilar to the present study, Mont et al¹⁷ also reported a lower but not significant difference in the frontal peak moment between the surface hip arthroplasty and control groups. We think that the 6- to 18-month follow-up period in their study could account for that difference because some of their patients had more than twice the time for recuperation compared with our patients. Nevertheless, this also shows that surface hip arthroplasty could possibly allow patients to return to a normative gait pattern faster than THA.

Effect of Biomechanical Reconstruction 

Some authors¹⁸ have proposed that specific characteristics of the surface hip arthroplasty prosthesis may ease the functional recuperation. Among these characteristics, the less invasive character of surface hip arthroplasty compared with the THA prosthesis is often mentioned.², ⁸, ¹¹ In surface hip arthroplasty, there is no stem in the medullar canal, and the femoral head is kept in place. Thus, surface hip arthroplasty is considered as a tissue-conserving alternative to THA, giving more possibilities for revision surgeries.

The second characteristic concerned the restoration of the hip biomechanics. Girard et al¹⁸ proposed that surface hip arthroplasty could allow a more precise reconstruction of the hip joint. Our results are in line with this suggestion because the restoration with respect to the contralateral hip was more similar for the surface hip arthroplasty subjects than for THA subjects (see table 2). The placement of the hip center of rotation is of particular importance because it modifies the abductor muscles lever arm and, consequently, influences the functional outcomes. It is generally accepted that the medialized placement of the center of rotation enhances the capacity of the abductor muscles to generate the moment of force.³¹, ³² However, this mechanical advantage was not taken into account in the estimation of the hip abductor moment of force. Consequently, this parameter could have been underestimated in both patient groups. Stagni et al³³ reported a mean error of 15% in the abductor-adductor moment of force curve with a 30-mm lateral mislocation of the hip center of rotation. Consequently, without minimizing the effect of this parameter on the kinetic analysis, we think that the medialized center of rotation positions reported here (1.5mm and 3.9mm in surface hip arthroplasty and THA groups, respectively) would have a small effect on the results.

Finally, the third characteristic is the larger femoral head component. It has been proposed that larger components reproduce a more anatomic fitting, similar to a normative hip joint.¹⁸ Consequently, it generates a more physiologic loading compared with smaller femoral heads.¹⁹ Therefore, we think that the characteristics of surface hip arthroplasty could have facilitated a normative sagittal gait pattern and could ease the return to a normative frontal gait pattern in the surface hip arthroplasty group.

However, further studies are needed to discriminate the effect of the bearing diameter, the presence of femoral stem, and hip biomechanics restoration on the gait pattern of THA and surface hip arthroplasty patients. Particularly, the effect of the femoral offset must be assessed because Asayama et al³¹ suggested that a larger femoral offset in THA would decrease the hip abductor muscle force required to walk. Furthermore, the generalization of the results should benefit from a larger sample size of patients, and a longer follow-up is needed to investigate the time needed to fully recover abductor muscle strength. Finally, contrary to the biomechanic gait evaluation, the WOMAC and Merle d'Aubigné-Postel scales failed to show differences between groups. Consequently, it underlines the distinctive and crucial role of the biomechanic analysis to determine prosthetics' specific functional impairments and mechanical adaptations.

Musculoskeletal Rehabilitation Considerations 

In regard to the functional and muscular deficit reported up to 2 years after hip arthroplasty,¹³, ¹⁴, ¹⁵ some authors¹⁵, ³⁴, ³⁵, ³⁶ have questioned the efficiency of the rehabilitation program after hip surgery. Despite mechanical advantages related to the prosthetic characteristics of surface hip arthroplasty, the hip abductor strength deficit and the deficient gait pattern in the frontal plane shown by the THA and surface hip arthroplasty groups support the re-evaluation of the standard rehabilitation intervention after hip arthroplasty.

Many authors¹⁵, ²⁶, ²⁷, ³⁴ suggested increasing the actual standard 2- to 3-month rehabilitation program to a systematic longer follow-up. In regard to the results reported in the present study, it could have been particularly interesting to continue the rehabilitation program to ensure the complete regain of the hip abductor strength as well as a normative gait pattern. Furthermore, a complete functional recovery could help prevent falls and injuries during more challenging activities such as physical activities and sports. Indeed, among numerous training programs proposed to restore the functional level and to improve mobility after THA, training programs focusing on muscle strengthening and especially weight-bearing tasks seem to provide the best results both in terms of postural stability and gait efficiency.¹⁵ Sashika et al³⁵ found significantly greater improvements in hip abductor muscle strength and gait parameters (walking speed and cadence) after low-resistance strengthening plus 1-legged stance exercises than after training only based on low-resistance exercises.

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Conclusions 

The present gait analysis showed that surface hip arthroplasty patients return to a normative sagittal gait pattern, whereas THA patients develop an adaptive strategy that could enhance the control of the COM and increase the energy generation during the swing phase. The specific characteristics of surface hip arthroplasty could play a major role in this fast return to normative gait pattern. In the frontal plane, the gait pattern modification observed in both groups is possibly related to the hip abductor muscles that did not completely reach the strength of the contralateral hip. This emphasizes the critical attention that must be paid to the hip muscle strength recovery after hip arthroplasty.

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Acknowledgment 

We thank Muthu Ganapathi, FRCS (Orth) for the radiographic analysis.

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• a Zimmer, PO Box 708, 1800 W Center St, Warsaw, IN 46581.
• b Clinical Measurement Corp, 12 A Chestnut, Ridgewood, NJ 07450.
• c VICON, 14 Minns Business Park, West Way, Oxford, UK OX2 0JB.
• d AMTI, model OR6-5; Advanced Mechanical Technology, 176 Waltham St, Watertown, MA 02472.
• e PG Drives Technology, 100 Airspeed Rd, Christchurch, Dorset, UK BH23 4HD.