Keywords

Introduction

Humans are known to have started bipedal locomotion prior to the complex brain being developed. The human musculoskeletal system has evolved both anatomically and biomechanically to provide optimal stability during locomotion. The hip joint is one of the most heavily loaded joints taking the peak average joint contact force of about 2.5 times of the body weight at the walking speed of 1.5 m/s (van den Bogert et al. 1999). It is therefore considered the most crucial joint in terms of balance and stability, particularly when in an upright, dynamic, weight-bearing position. During weight bearing, the hip joint transfers the full upper body load to the lower extremities and as a consequence is surrounded by major muscle groups providing the required support. The hip joint is comprised of the head of the femur and the acetabulum of the pelvis. It is a ball- and socket-type joint, which, providing the additional benefit of mobility in several planes, increases adaptive stability. In knowing this, it can be assumed that a diseased and/or painful hip joint could significantly restrict locomotion, thereby affecting day-to-day activities of the individual with arthritis. The most common and successful surgical treatment for end-stage hip joint disease is THA. In fact, based on the recent statistics, THA is performed annually on approximately one million patients worldwide (OECD 2015), with the USA itself accounting for about 330,000 cases per year (Centers For Disease Control and Prevention 2010). In THA, the diseased degenerated joint is completely replaced by artificial implants, which are said to perform similarly in terms of biomechanical function to a healthy joint. However, this claim is backed by empirical data with little information related to patient gait mechanics post-THA. One of the main reasons for this lack of objective data is that current clinical practice utilizes time and cost-effective, subjective assessment methods, which rely solely on a patient’s perception of their pain and functional independence, over more objective gait assessments. The main reason is that gait assessment systems are typically large and expensive and require technical training and expertise to be used properly.

State of the Art

Modern gait assessment methods are not only capable of providing accurate information regarding the biomechanical aspects of gait but are also becoming both more affordable and more accessible. Current objective gait assessment methods range from laboratory-based methods, which include high-speed cameras, force platforms, and video fluoroscopy, to portable ambulatory systems, which utilize 3-D inertial sensors and pressure insoles. Furthermore, physical activity can now be assessed both quantitatively and qualitatively, using portable and cost-effective triaxial accelerometers. The biomechanical function of the healthy hip joint is well studied. However, alterations in gait mechanics following the onset of hip joint pathologies are under continual debate, with several studies publishing contradictory results. Most notable, mid- to long-term postoperative gait studies have shown no significant effect of the type of surgical approach in relation to gait outcome, whereas early postoperative studies have reported otherwise and hence such strong conclusions cannot automatically be drawn.

Gait Assessment Methods

There are a variety of human motion analysis systems available from in vitro to in vivo. Among the existing methods, lab-based methods consisting of both high frame rate video cameras and high-precision force sensors embedded in a platform are considered to be the “gold standard.” Retroreflective markers are placed on anatomical locations, and joint movement is tracked by infrared cameras, providing a full 3-D map of joint kinematics. Furthermore, force platforms, depending on size, calculate the involved ground forces for one up to ten consecutive steps.

Ambulatory gait assessment (AGA) methods consist of inertial sensors and pressure insoles. Inertial sensors are miniature devices consisting of 3-D accelerometers, gyroscopes, and magnetometers (Tao et al. 2012), while pressure insoles have flexible sensors attached to them. In terms of acquired data, AGA provides similar information to lab-based equivalents but is far more economical in terms of cost, space, and time. It also provides the freedom to assess individuals in their free-living environment, testing not just the quality but also the quantity of activity. However, in the above-described methods, the markers/sensors are placed on the body surface and therefore subject to external environmental factors which could alter the placement of the markers/body worn sensors leading to errors in the collected data (Della Croce et al. 1999). Furthermore, as the hip is a deeply placed joint in the body, soft tissue artifacts may appear in the data. However, mathematical algorithms are available to minimize these artifacts. Another purely functional drawback of the marker-/body-worn sensor systems is that it has been shown to have error in terms of accurately locating the hip joint center, a crucial parameter, which can result in kinematic assessment inaccuracies (Fiorentino et al. 2016).

Video fluoroscopy is another kinematic assessment method. It utilizes a series of x-ray images, which are recorded as video frames. This method provides accurate in vivo information of the joint motion in real time without any interference from artifacts produced by skin or soft tissues (Bejjani et al. 1992). It has been found to be a method for accurate measurement of the hip joint center, compared to the marker system (Fiorentino et al. 2016). Note, however, that video fluoroscopy, even in providing a more accurate measure of kinematics, is limited to research use only. This is due to the continuous exposure to radiation during repeated recordings exceeding limitations for the clinical purposes.

Another lab-based assessment method is electromyography (EMG). EMG captures the electrical signals generated by the neuromuscular activation of the skeletal muscle. For hip joint assessment, EMG plays an important role as the hip joint is surrounded by major muscle groups which play an important role in the stability, balance, and smooth mobility of the joint. Muscle imbalance around a joint can notably lead to various compensatory gait alterations, further backing the importance of EMG assessment.

Hip Joint Gait Mechanics

Biomechanically, a healthy hip joint is known to provide more stability than mobility. During different phases of gait, the hip joint adapts and rotates to maintain balance. The majority of motion occurs in the sagittal plane, approximately 30°, with a maximum hip flexion of about 25–30° and a maximum extension of about 8–10°. Relative motion in the frontal and transverse planes is approximately 10–12° and 6–8°, respectively (Fig. 1). Upward and downward pelvic tilt has also been shown to add 3–4° of both flexion and extension to the hip joint, respectively (Roberts 2010). Furthermore, the forces applied by the surrounding muscles (moment/torque) adaptively change during walking in both sagittal and frontal planes to maintain balance. In the sagittal plane, a peak in the external extension moment can be seen on initial ground contact with a gradual transition into a flexion moment continuing until the terminal stance (Fig. 1). In contrast, in the frontal plane, the external abduction moment dominates throughout the stance phase. Lastly, the external moment in the transverse plane showed a relatively small external rotation peak at the initial contact, followed by a small internal rotation peak during the mid-stance which is then observed to gradually transition to either an external rotation moment or to a neutral position during terminal stance. Power at a joint is the product of the muscle moment and its angular velocity. Power assessment at hip joint showed four peaks at around 5, 45, 60, and 95% of the gait cycle (GC), the peaks during the stance phase shows power absorption, and the peaks during the swing phase shows power generation (Fig. 3). The ground reaction force (GRF) vector has been shown to move progressively from an anterior to posterior position as the gait proceeds from initial contact to terminal stance.

Fig. 1
figure 1

Range of motion and internal moments at the hip joint during level walking in ten healthy adults (age 30 ± 4 years, BMI 23.9 ± 2.4 kg/m2). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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Pathologic Versus Instrumented Hip Gait Mechanics

An arthritic hip joint can be diagnosed via clinical and radiographic hallmarks. Clinical hallmarks of hip osteoarthritis include pain, tenderness, stiffness, and loss of flexibility. Radiographic hallmarks include nonuniform joint space narrowing, osteophyte formation at the joint line, and development of subchondral sclerosis and cysts. Notably, there are no such hallmarks for gait assessment. However, as it is known that gait patterns vary between individuals, one cannot expect a completely predefined gait pattern for a given body type and never mind a specific pathology. Nonetheless, one can benchmark alterations in gait as would be seen in the majority of patients, both with and without individualistic variances. Furthermore, it is always beneficial to test other weight-bearing joints, on both the ipsilateral and contralateral sides, to check for adaptations following the hip deformity (Watelain et al. 2001; Yoshimoto et al. 2005; Foucher and Wimmer 2012). This is because the weight-bearing joints work in a closed kinetic chain during weight-bearing activities and are therefore directly or indirectly affected by any alterations. The following sections compare the gait mechanics of pathological and instrumented hip joints. Arthritis is used as an example pathology as most hip pathologies have a tendency to result in arthritis, at which point total hip replacement becomes the treatment of choice.

Daily Physical Activity

Patients with hip arthritis are expected to see a noticeable reduction in their daily physical activity level, due to pain and discomfort. Furthermore, an overall reduced physical activity leads to muscle wasting around the affected hip joint. Quantitatively, based on accelerometer activity measured in counts per minute (cpm), healthy elderly adults aged above 60 years show an average activity level of 200–400 cpm (Hagstromer et al. 2007), (Evenson et al. 2012). For patients with hip arthritis, the physical activity level is seen to decline to 100–200 cpm (Harding et al. 2014). Following THA, short-term follow-up studies report no improvement in physical activity level (Harding et al. 2014); however, long-term follow-up studies show significant improvements (Lubbeke et al. 2014). It should be noted, however, that the energy cost of moderate to vigorous physical activity has shown a significant increase following THA (Lin et al. 2013).

Spatiotemporal

Patients with hip arthritis generally experience a reduction in walking speed, 1.01–1.08 m/s compared with 1.2–1.3 m/s for healthy adults, as a consequences of a general reduction in stride length (1.13–1.17 m compared to 1.2–1.3 m), in single support time (35% of cycle compared with 37%), and in cadence (103–110 steps/min compared to 115–120 steps/min). Furthermore, step width is seen to be abnormally wide (0.15–0.19 m compared to 0.08–0.12 m) due to the inherent lateral positioning of the feet (Shrader et al. 2009). An increased step width also helps to provide a larger base of support, probably to accommodate for the hip’s diminished postural strategy. The reduced walking speed is directly related to pain and discomfort. It follows that a reduction in stride length occurs as a result of a restricted freedom of motion and that both a lower cadence and reduced support time are as a result of patients consciously minimizing the frequency and duration of loading.

Following THA, patient walking speeds (1.2–1.4 m/s), stride lengths (1.3–1.4 m), single support times (37.2 (% of cycle)), and cadence (112–119 steps/min) increase significantly. Furthermore, the step width decreases to 0.1–0.08 m (Shrader et al. 2009), representing not only an improved gait pattern but also an improved hip function, i.e., an enhanced freedom of motion. Do note, however, that despite the reported improvements compared to preoperative values, the walking speed in patients with THA is still generally slower in comparison to their healthy counterparts. Assuming no biomechanical issues remain, this could be due to apprehension, fear of falling, and/or a longer than expected period of acclimatization.

Kinematics

The active and passive range of motion of an arthritic hip is shown to be reduced in all planes. An arthritic hip is observed to stay in a relatively flexed position over most of the gait cycle, reporting peak hip flexion between 25 and 30° and a peak extension as low as 0−2°. Following THA , this altered motion in the sagittal plane is noted to have improved; however, it is still seen to be significantly lower than a healthy hip joint (Fig. 2).

Fig. 2
figure 2

Hip kinematics and kinetics at sagittal, frontal, and transverse planes from ten patients with severe arthritis, before (black) and 1 year after total hip arthroplasty (red). Average age and BMI of the patient group were 51.03 ± 4.4 years and 28.78 ± 5.4 kg/m2, respectively. Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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In the frontal plane, overall motion at an arthritis hip is seen to be reduced. Unlike a healthy hip, an arthritic hip begins the initial contact in a neutral or abducted position and either stays in the abducted position or moves toward adduction between 0 and 6° at approximately 20–25% of the gait cycle. The peak abduction angle during toe-off stays between 0 and −10°. Following THA, an increased adduction motion is observed (approximately 8–10°) returning a near normal pattern of motion in the frontal plane (Fig. 2).

Lastly, in transverse plane, an arthritic hip stays predominantly in the internally rotated position, with an external rotation (2–5°) only being reported at the initial contact and terminal swing phases of the gait cycle (Fig. 2). Following THA, an improvement is expected in the transverse plane motion as it is mostly a secondary motion inherent to the hip muscles operating in the sagittal and frontal planes. However, the observed improvement in range of motion is still not comparable to a healthy hip. Overall, a large variability is seen in motion in all three planes of patients with hip arthritis which continues to exist following THA. This would suggest each patient could show a unique adaptation strategy.

Kinetics

The forces generated by muscles are not only necessary to support the body in an upright position but are also necessary for forward propulsion of the body by continuous redistribution of muscle power, between concentric and eccentric contractions, around the trunk and leg muscles. In hip arthritis, a significant reduction in muscle properties is seen on the affected side, compared to the contralateral asymptomatic side (Barrett et al. 2013). In particular, arthritic hip muscles, along with the corresponding knee extensors, have shown a reduction in overall muscle strength for both concentric and eccentric contractions (Loureiro et al. 2013). Muscle size and density have also been observed to have been adversely affected, as has the muscle activation of hip muscles on the affected side (Loureiro et al. 2013). All of the above structural and functional muscle deficiencies lead to an overall reduction in cumulative force produced by the muscles, which would further lead to patient gait asymmetry and the development of compensatory gait strategies. For example, the Trendelenburg gait pattern seen in hip pathology patients is mainly due to the weakness of the hip abductors, specifically, the gluteus medius and minimus.

There are quantitative differences in the joint moments of healthy, arthritic, and instrumented hip (Table 1). In contrast with healthy individuals, patients with hip arthritis are shown to walk with reduced muscle moments in all three planes, which suggest an overall reduced muscle function in an arthritic hip. Furthermore, the altered external rotation moment peak is specifically of interest, as the small peak is seen to occur at the early stance and continues to stay until the early swing, unlike the healthy hip. This pattern is unfortunately noted to continue 1 year following THA (Fig. 2). Also, the inability to extend and adduct the hip may, in long term, lead to flexion and abduction contractures, as is evident in patients with severe hip arthritis. The reduced joint power peaks for patients with arthritis confirm the reduced power generation and absorption at the diseased hip and the adjacent joints on the affected side (Fig. 3). The reduced power absorption during the initial and terminal stance, as well as a reduced power generation during push-off, results in an overall reduced hip moment in the sagittal plane.

Table 1 Range of available hip joint moments (nm/kg) for healthy, arthritic, and instrumented hip joints. Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN
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Fig. 3
figure 3

Joint power curves at the hip, knee, and ankle joint at the sagittal plane for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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Following THA, studies have shown a significant increase in both flexion and abduction moments and corrective external rotation moment pattern in the early stance phase. Regained muscle power, as a result of intensive and focused rehabilitation protocols, is one of the reasons why patients with THA are known for fast recovery.

Neighboring Joint Strategies

The lower extremities and the trunk work as a closed kinetic chain during weight-bearing activities and consequently impact one another if their mechanics are altered. As a result, a disruption in hip joint mechanics can have an adverse effect on the spine, pelvis, knee, and ankle joints, both on the ipsilateral side (directly) and even on the contralateral side (indirectly), as has been observed in patients with severe hip arthritis.

The pelvis, being the closest neighboring segment, shows the most alteration in hip pathologies. In particular, a restricted hip motion has been shown to increase the pelvic tilt and pelvic obliquity as a compensatory mechanism (Fig. 4). Pelvic tilt is the anterior and posterior motion of the pelvis in the sagittal plane, while pelvic obliquity is the lateral upward and downward motion of the pelvis in the frontal plane. During walking normal pelvic tilt is between 2° and 8°, pelvic obliquity between 2° and 4°, and pelvic rotation between 8° and 10° (Perry and Burnfield 2010).

Fig. 4
figure 4

Pelvis motion for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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The increased pelvic tilt and obliquity shortens the moment arm between the hip and the upper body center of mass, which results in extension of the hip joint, initiating push-off at the end of the stance phase (Yoshimoto et al. 2005). This pelvic strategy further leads to an increased trunk inclination, toward the sound side, which is seen to have a direct impact on the lumbar spine resulting in lumbar dysfunction. Furthermore, hip flexion contracture and an increased lumbar lordosis are seen as a compensatory mechanism to initiate hip extension. Pelvic incidence is another parameter which is seen to be significantly higher in patients with hip arthritis and is itself strongly correlated to pelvic tilt, lumbar lordosis, and sacral slope (Yoshimoto et al. 2005). Pelvic incidence is calculated by adding the pelvic tilt and the sacral slope.

For the knee joint, two primary muscle groups control the motion. These primary muscle groups are two jointed and cross both the hip and knee joints. Consequently, muscle weakness surrounding the hip could have direct effect on muscle control at the knee joint as has been witnessed in patients with hip arthritis (Fig. 3). Involved muscles include semitendinosus, semimembranosus, and long head of biceps femoris of the hamstring group, posteriorly, and rectus femoris part of the quadriceps, anteriorly.

In patients with hip arthritis, the knee motion in sagittal plane is reduced, while in frontal and transverse plane, it is noticeably altered (Fig. 5). The altered varus and inverted position of the knee during the early to late swing phase are seen to persist 1 year postoperatively and continue to stay significantly different from the healthy knee motion. A noticeable reduction in joint moment is also seen in hip arthritis, which is seen to improve 1 year following THA. However, compared to the healthy knee, this reported improvement is still not comparable.

Fig. 5
figure 5

Knee motion and moment for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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In terms of joint power in the sagittal plane, patients with hip arthritis show 50–60% less energy generation and absorption compared to the healthy knee (Fig. 3). The reduced power at the knee is due to the altered functions of the muscles connecting between the diseased hip and knee. For example, during mid-stance, when the knee straightens to take the load, a reduced power generation is seen as a result of the reduced hip extension. Furthermore, during push-off, patients with arthritis demonstrate a 40–45% reduction in peak energy absorption to assist the push-off with restricted hip extension. Lastly, at the late swing phase, a 30–35% reduction in energy absorption can be seen in the fourth peak, representing a reduced peak swing speed. The frontal and transverse planes also are shown to absorb less power during the push-off (Watelain et al. 2001). This is a compensatory strategy to limit hip abduction in order to maintain the lower limb joint alignment.

Finally, at the ankle joint, patients with hip arthritis show a significantly reduced motion seen in all three planes. However, muscle moment showed no difference in the sagittal plane, while significantly increased and decreased moments are found at the frontal and transverse planes, respectively (Fig. 6). The power generation during push-off is seen to be low compared to the healthy ankle; this could be a compensation strategy to facilitate push-off with weak hip extension. The power generation peak is seen to improve following THA but stays relatively low compared to the healthy knee (Fig. 3).

Fig. 6
figure 6

Ankle motion and moment for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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In relation to THA , studies have demonstrated notable improvements over initial gait alterations; however, gait mechanics typically do not return to normal (Horstmann et al. 2013). THA is found to be effective in reducing the lower back pain symptoms; however, gait mechanics at pelvis and spine have not been seen to return to normal (Ben-Galim et al. 2007; Foucher and Wimmer 2012). The alterations at knee and ankle joint mechanics are seen to persist up to 6–12 months postoperatively (Reardon et al. 2001; Foucher and Wimmer 2012) (Beaulieu et al. 2010). Consequently, one can predict that the preoperative gait mechanics in the neighboring joints, including those bilaterally, may lead to arthritic changes due to consistent abnormal loading. Notably, there are studies reporting a lateral patellar tilt on the ipsilateral knee, due to the internally rotated position of the hip, with an increased adduction moment and the development of medial tibiofemoral arthritis on the affected side as a result of the altered axial alignment (Akiyama et al. 2016; Umeda et al. 2009).

The discussed outcome of THA is seen to be consistent with the various surgical approaches used, including anterior, posterior, direct lateral, anterolateral, and posterolateral (Queen et al. 2014; Wesseling et al. 2016). However, inconsistencies can be found in studies comparing early postoperative outcomes (Queen et al. 2011; Barrett et al. 2013; Madsen et al. 2004). From an anatomical perspective, the anterior approach should help with a faster recovery of gait as the hip muscles are preserved during the surgery. In contrast, lateral or posterior approaches result in the muscles being dissected to access the joint, and the recovery of muscle strength following these procedures is notably longer. It can also be seen that the minimally invasive THA is shown to have better gait recovery than the conventional method (Weber et al. 2014).

Stair Ascending and Descending Biomechanics

Joint mechanics during stair activity should be considered while assessing the outcome of THA . It can be said that walking up and down stairs requires a higher degree of freedom of motion of the hip joint (Table 2). Furthermore, good muscle control especially of the major two jointed muscles around the hip and knee joint is important for good power and balance (Andriacchi et al. 1980). The power generation is seen to be significantly higher during stair ascending than compared to descending (Fig. 7). This is confirmed by the noticeably higher muscle moments in the sagittal plane, reaching up to 1 nm/kg while ascending, in comparison to less than 0.5 nm/kg during descent, in healthy adults (Fig. 8). However, in the frontal plane, stair descent uses a muscle force >1 nm/kg.

Table 2 Range of motion during stair ascent and descent at self-selected speed in healthy, arthritic, and instrumented hip joint
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Fig. 7
figure 7

Hip joint external moments curve during ascend (top) and descend (bottom) of stairs for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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Fig. 8
figure 8

Hip joint power during ascend and descend of stairs for healthy adults (gray), patients with hip arthritis (black), and patients following 1-year total hip arthroplasty (red). Results are from the Motion Analysis Laboratory, at Mayo Clinic, Rochester, MN

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In hip arthritis, difficulty with stair climbing has been consistently reported, due to the weakness of the major hip muscles leading to reduced muscle moments (Fig. 7) and an altered power generation (Fig. 8). Power generation is seen to reduce significantly in hip arthritis which would lead to compensation strategies to achieve the range of motion required to initiate stair climbing. The inability of muscles to produce enough power for the increased range of motion required during stair climbing is compensated by a reduced stair climbing speed in these patients. All of the above results in patients adopting altered angular and loading strategies when both ascending and descending stairs, which may lead to bilateral asymmetries. Following THA, joint muscle moment and power are seen to improve significantly, returning the power generation to near normal requirements for stair climbing. However, the improvement is not comparable to a healthy hip. Note that the transverse pain moments, though significantly lower compared to the other two planes, could have some effect due to the altered pattern during both ascending and descending stairs.

Future Work

Research suggests that investigating the extent of remaining preoperative gait patterns at each joint would help improve rehabilitation, by reducing preoperative gait abnormalities as early as possible. Large-scale studies are therefore needed, assessing all lower extremity joints both pre- and postoperatively for both short- and long-term THA outcomes. There is also a need for a simplified objective gait score, assessing the severity of gait alterations in patients with hip arthritis which would assist clinicians in planning treatments, accordingly.

Cross-References