Joint contact stresses calculated for acetabular dysplasia patients using discrete element analysis are significantly influenced by the applied gait pattern

Providing a computationally derived, mechanically optimised target correction during preoperative planning can improve joint contact mechanics of hip dysplasia treated with periacetabular osteotomy

AIM

Preoperative identification of acetabular corrections that optimally improve joint stability and reduce elevated contact stresses could further reduce osteoarthritis progression in patients with hip dysplasia who are treated with periacetabular osteotomy (PAO). The purpose of this study was to investigate how providing patient-specific, mechanically optimal acetabular reorientations to the surgeon during preoperative planning affected the surgically achieved correction.

METHODS

Preoperative CT scans were used to create patient-specific hip models for 6 patients scheduled for PAO. A simulated acetabular fragment was extracted from the preoperative pelvis model and computationally rotated to simulate candidate acetabular reorientations. For each candidate, discrete element analysis was used to compute contact stresses during walking, which were summed over the gait cycle and scaled by patient age to obtain chronic contact stress-time exposure. The ideal patient-specific reorientation was identified using a cost function that balances minimising chronic stress exposures and achieving surgically acceptable acetabular coverage angles. The optimal reorientation angles and associated contact mechanics were provided to the surgeon preoperatively. After PAO was performed, a model of the surgically achieved correction was created from a postoperative CT scan. Radiographic coverage and contact mechanics were compared between preoperative, optimal, and surgically achieved orientations.

RESULTS

While surgically achieved reorientations were not significantly different from optimal reorientations in radiographically measured lateral (p = 0.094) or anterior (p = 0.063) coverage, surgically achieved reorientations had significantly (p = 0.031) reduced total contact area compared to optimal reorientations. The difference in lateral coverage and peak chronic exposure between surgically achieved and optimal reorientations decreased with increasing surgeon experience using the models (R² = 0.758, R2 = 0.630, respectively).

CONCLUSIONS

Providing hip surgeons with a patient-specific, computationally optimal reorientation during preoperative planning may improve contact mechanics after PAO, which may help reduce osteoarthritis progression in patients with hip dysplasia.

The Effects of Residual Femoral Deformity on Computed Contact Mechanics in Patients Treated With In Situ Fixation for Slipped Capital Femoral Epiphysis

OBJECTIVE

In situ fixation for treatment of slipped capital femoral epiphysis (SCFE) can stabilize the epiphysis and prevent further joint deformation but often leaves residual deformity that may adversely affect intra-articular contact mechanics. The purpose of this study was to investigate the relationship between residual deformity and contact mechanics in the post-SCFE hip.

METHODS

Patient-specific hip models were created for 19 patients with SCFE treated with in situ fixation. For each model, discrete element analysis was used to compute cumulative acetabular and femoral contact stress exposure during a walking gait cycle. Slip severity was evaluated for each patient using the two-dimensional Southwick angle and a novel three-dimensional (3D) assessment of multiplanar femoral deformity (3D slip angle).

RESULTS

Of the SCFE cases, 2/7 mild (Southwick angle ≤30 degrees) had peak cumulative femoral exposures equivalent to that of severe (Southwick angle ≥60 degrees) cases. Severe SCFE cases had higher peak ( P = 0.015) and mean ( P = 0.028) femoral contact stress exposure and lower cumulative femoral contact area ( P = 0.003) than mild (Southwick angle ≤30 degrees) SCFE cases. Mean femoral contact stress exposure was also higher in severe SCFE cases than in moderate SCFE cases ( P = 0.027). Acetabular and femoral contact mechanics metrics typically demonstrated stronger correlations with 3D slip angle than two-dimensional Southwick angle.

CONCLUSIONS

Increased slip severity adversely impacts intra-articular femoral contact mechanics. Contact mechanics metrics demonstrate higher correlations with 3D slip angle, indicating that this novel measurement may better describe global deformity and its relationship to intra-articular mechanics; however, the modest strength of these correlations may also imply that global impingement-generating deformity is not the primary factor driving contact mechanics in the post-SCFE hip.

CLINICAL RELEVANCE

Greater slip severity adversely impacts contact mechanics in the post-SCFE hip. However, focal regions of high contact stress were seen even in mild SCFE deformities, suggesting some type of deformity correction should be considered even for mild slips to alleviate secondary impingement, address focal incongruities, and reduce osteoarthritis development/progression.

Radiographically successful periacetabular osteotomy does not achieve optimal contact mechanics in dysplastic hips

BACKGROUND

Optimal correction of hip dysplasia via periacetabular osteotomy may reduce osteoarthritis development by reducing damaging contact stress. The objective of this study was to computationally determine if patient-specific acetabular corrections that optimize contact mechanics can improve upon contact mechanics resulting from clinically successful, surgically achieved corrections.

METHODS

Preoperative and postoperative hip models were retrospectively created from CT scans of 20 dysplasia patients treated with periacetabular osteotomy. A digitally extracted acetabular fragment was computationally rotated in 2-degree increments around anteroposterior and oblique axes to simulate candidate acetabular reorientations. From discrete element analysis of each patient's set of candidate reorientation models, a mechanically optimal reorientation that minimized chronic contact stress exposure and a clinically optimal reorientation that balanced improving mechanics with surgically acceptable acetabular coverage angles was selected. Radiographic coverage, contact area, peak/mean contact stress, and peak/mean chronic exposure were compared between mechanically optimal, clinically optimal, and surgically achieved orientations.

FINDINGS

Compared to actual surgical corrections, computationally derived mechanically/clinically optimal reorientations had a median[IQR] 13[4-16]/8[3-12] degrees and 16[6-26]/10[3-16] degrees more lateral and anterior coverage, respectively. Mechanically/clinically optimal reorientations had 212[143-353]/217[111-280] mm2 more contact area and 8.2[5.8-11.1]/6.4[4.5-9.3] MPa lower peak contact stresses than surgical corrections. Chronic metrics demonstrated similar findings (p ≤ 0.003 for all comparisons).

INTERPRETATION

Computationally selected orientations achieved a greater mechanical improvement than surgically achieved corrections; however, many predicted corrections would be considered acetabular over-coverage. Identifying patient-specific corrections that balance optimizing mechanics with clinical constraints will be necessary to reduce the risk of osteoarthritis progression after periacetabular osteotomy.

Joint contact stress improves in dysplastic hips after periacetabular osteotomy but remains higher than in normal hips

AIM

The purpose of this study was to use computational modeling to determine if surgical correction of hip dysplasia restores hip contact mechanics to those of asymptomatic, radiographically normal hips.

METHODS

Discrete element analysis (DEA) was used to compute joint contact stresses during the stance phase of normal walking gait for 10 individuals with radiographically normal, asymptomatic hips and 10 age- and weight-matched patients with acetabular dysplasia who underwent periacetabular osteotomy (PAO).

RESULTS

Mean and peak contact stresses were higher (p < 0.001 and p = 0.036, respectively) in the dysplastic hips than in the matched normal hips. PAO normalised standard radiographic measurements and medialised the location of computed contact stress within the joint. Mean contact stress computed in dysplastic hips throughout the stance phase of gait (median 5.5 MPa, [IQR 3.9-6.1 MPa]) did not significantly decrease after PAO (3.7 MPa, [IQR 3.2-4.8]; p = 0.109) and remained significantly (p < 0.001) elevated compared to radiographically normal hips (2.4 MPa, [IQR 2.2-2.8 MPa]). Peak contact stress demonstrated a similar trend. Joint contact area during the stance phase of gait in the dysplastic hips increased significantly (p = 0.036) after PAO from 395 mm2 (IQR 378-496 mm2) to 595 mm2 (IQR 474-660 mm2), but remained significantly smaller (p = 0.001) than that for radiographically normal hips (median 1120 mm2, IQR 853-1444 mm2).

CONCLUSIONS

While contact mechanics in dysplastic hips more closely resembled those of normal hips after PAO, the elevated contact stresses and smaller contact areas remaining after PAO indicate ongoing mechanical abnormalities should be expected even after radiographically successful surgical correction.

Chronically elevated contact stress exposure correlates with intra-articular cartilage degeneration in patients with concurrent acetabular dysplasia and femoroacetabular impingement

Hip dysplasia is known to lead to premature osteoarthritis. Computational models of joint mechanics have documented elevated contact stresses in dysplastic hips, but elevated stress has not been directly associated with regional cartilage degeneration. The purpose of this study was to determine if a relationship exists between elevated contact stress and intra-articular cartilage damage in patients with symptomatic dysplasia and femoroacetabular impingement. Discrete element analysis was used to compute hip contact stresses during the stance phase of walking gait for 15 patients diagnosed with acetabular dysplasia and femoral head-neck offset deformity. Contact stresses were summed over the duration of the walking gait cycle and then scaled by patient age to obtain a measure of chronic cartilage contact stress exposure. Linear regression analysis was used to evaluate the relationship between contact stress exposure and cartilage damage in each of six acetabular subregions that had been evaluated arthroscopically for cartilage damage at the time of surgical intervention. A significant correlation (R2  = 0.423, p < 0.001) was identified between chondromalacia grade and chronic stress-time exposure above both a 1 MPa damage threshold and a 2 MPa-years accumulated damage threshold. Furthermore, an over-exposure threshold of 15% regional contact area exceeding the 1 and 2 MPa-years threshold values resulted in correct identification of cartilage damage in 83.3% (55/66) of the acetabular subregions loaded during gait. These results suggest corrective surgery to alleviate impingement and reduce chronic contact stress exposures below these damage-inducing thresholds could mitigate further cartilage damage in patients with hip dysplasia.

Effect of modeling femoral version and head-neck offset correction on computed contact mechanics in dysplastic hips treated with periacetabular osteotomy

While correction of dysplastic acetabular deformity has been a focus of both clinical treatment and research, concurrent femoral deformities have only more recently received serious attention. The purpose of this study was to determine how including abnormalities in femoral head-neck offset and femoral version alter computationally derived contact stresses in patients with combined dysplasia and femoroacetabular impingement (FAI). Hip models with patient-specific bony anatomy were created from preoperative and postoperative CT scans of 20 hips treated with periacetabular osteotomy and femoral osteochondroplasty. To simulate performing only a PAO, a third model was created combining each patient's postoperative pelvis and preoperative femur geometry. These three models were initialized with the femur in two starting orientations: (1) standardized template orientation, and (2) using patient-specific anatomic landmarks. Hip contact stresses were computed in all 6 model sets during an average dysplastic gait cycle, an average FAI gait cycle, and an average stand-to-sit activity using discrete element analysis. No significant differences in peak contact stress (p = 0.190 to 1), mean contact stress (p = 0.273 to 1), or mean contact area (p = 0.050 to 1) were identified during any loading activity based on femoral alignment technique or inclusion of femoral osteochondroplasty. These findings suggest that presence of abnormal femoral version and/or head-neck offset deformities are not themselves predominant factors in intra-articular contact mechanics during gait and stand-to-sit activities. Inclusion of modified movement patterns caused by these femoral deformities may be necessary for models to adequately capture the mechanical effects of these clinically recognized risk factors for negative outcomes.

Acetabular Edge Loading During Gait Is Elevated by the Anatomical Deformities of Hip Dysplasia

Developmental dysplasia of the hip (DDH) is a known risk factor for articular tissue damage and secondary hip osteoarthritis. Acetabular labral tears are prevalent in hips with DDH and may result from excessive loading at the edge of the shallow acetabulum. Location-specific risks for labral tears may also depend on neuromuscular factors such as movement patterns and muscle-induced hip joint reaction forces (JRFs). To evaluate such mechanically-induced risks, we used subject-specific musculoskeletal models to compare acetabular edge loading (AEL) during gait between individuals with DDH (N = 15) and healthy controls (N = 15), and determined the associations between AEL and radiographic measures of DDH acetabular anatomy. The three-dimensional pelvis and femur anatomy of each DDH and control subject were reconstructed from magnetic resonance images and used to personalize hip joint center locations and muscle paths in each model. Model-estimated hip JRFs were projected onto the three-dimensional acetabular rim to predict instantaneous AEL forces and their accumulative impulses throughout a gait cycle. Compared to controls, subjects with DDH demonstrated significantly higher AEL in the antero-superior acetabulum during early stance (3.6 vs. 2.8 × BW, p ≤ 0.01), late stance (4.3 vs. 3.3 × BW, p ≤ 0.05), and throughout the gait cycle (1.8 vs. 1.4 × BW^*s, p ≤ 0.02), despite having similar hip movement patterns. Elevated AEL primarily occurred in regions where the shallow acetabular edge was in close proximity to the hip JRF direction, and was strongly correlated with the radiographic severity of acetabular deformities. The results suggest AEL is highly dependent on movement and muscle-induced joint loading, and significantly elevated by the DDH acetabular deformities.

Effects of Hip Abductor Strengthening on Musculoskeletal Loading in Hip Dysplasia Patients after Total Hip Replacement

Hip dysplasia patients after total hip replacement show worse functional performance compared to primary osteoarthritis patients, and unfortunately there is no research on muscle and joint loads that would help understand rehabilitation effects, motor dysfunctions and failure events. We tested the hypothesis that a higher functional improvement in hip dysplasia patients who received hip abductor strengthening after hip replacement, would result in different gait function and musculoskeletal loads during walking compared to patients who performed standard rehabilitation only. In vivo gait analysis and musculoskeletal modeling were used to analyze the differences in gait parameters and hip and muscle forces during walking between the two groups of patients. We found that, in a functional scenario of very mild abnormalities, the patients who performed muscle strengthening expressed a more physiological force pattern and a generally greater force in the operated limb, although statistically significant in limited portions of the gait cycle, and likely related to a higher gait speed. We conclude that in a low-demand task, the abductor strengthening program does not have a marked effect on hip loads, and further studies on hip dysplasia patients would help clarify the effect of muscle strengthening on loads.

Effect of simulated rehabilitation on hip joint loading during single limb squat in patients with hip dysplasia

Rehabilitation for patients with developmental dysplasia of the hip (DDH) addresses modifiable factors in an effort to reduce symptoms and prevent or delay the development of osteoarthritis, yet its effect on joint mechanics remains unknown. Our objective was to establish how rehabilitation (muscle strengthening and movement training), simulated with a musculoskeletal model and probabilistic analyses, alters hip joint reaction forces (JRF) in patients with DDH during a single limb squat. In four patients with DDH, hip abductor strengthening was simulated by increasing the maximum isometric force value between 0 and 32.6% and movement training was simulated by decreasing the hip adduction angle between 0 and 10° relative to baseline. 2,000 Monte Carlo simulations were performed separately to simulate strengthening and movement training, from which 99% confidence bounds and sensitivity factors were calculated. Our results indicated that simulated movement training aimed at decreasing hip adduction had a substantially larger influence on hip JRF than strengthening, as indicated by 99% confidence bounds of the resultant JRF (0.88 ± 0.55 xBW vs. 0.31 ± 0.12 xBW, respectively). Relative to baseline, movement training that resulted in a 10° decrease in hip adduction decreased the resultant JRF by 0.78 ± 0.65 xBW, while strengthening the abductors by 17.6% increased resultant JRF by 0.18 ± 0.06 xBW. To our knowledge, these results are the first to provide evidence pertaining to the effect of rehabilitation on joint mechanics in patients with DDH and can be used to inform more targeted interventions.

Discrete element and finite element methods provide similar estimations for hip joint contact mechanics during walking gait

Finite element analysis (FEA) provides a powerful approach for estimating the in-vivo loading characteristics of the hip joint during various locomotory and functional activities. However, time-consuming procedures, such as the generation of high-quality FE meshes and setup of FE simulation, typically make the method impractical for rapid applications which could be used in clinical routine. Alternatively, discrete element analysis (DEA) has been developed to quantify mechanical conditions of the hip joint in a fraction of time compared to FEA. Although DEA has proven effective in the estimation of contact stresses and areas in various complex applications, it has not yet been well characterised by its ability to evaluate contact mechanics for the hip joint during gait cycle loading using data from several individuals. The objective of this work was to compare DEA modelling against well-established FEA for analysing contact mechanics of the hip joint during walking gait. Subject-specific models were generated from magnetic resonance images of the hip joints in five asymptomatic subjects. The DEA and FEA models were then simulated for 13 loading time-points extracted from a full gait cycle. Computationally, DEA was substantially more efficient compared to FEA (simulation times of seconds vs. hours). The DEA and FEA methods had similar predictions for contact pressure distribution for the hip joint during normal walking. In all 13 simulated loading time-points across five subjects, the maximum difference in average contact pressures between DEA and FEA was within ±0.06 MPa. Furthermore, the difference in contact area ratio computed using DEA and FEA was less than ±6%.

Recovery of Lower Extremity Function in the Initial Year Following Periacetabular Osteotomy: A Single Subject Analysis

BACKGROUND

Little is known regarding how lower extremity joint function recovers in the 6 months following periacetabular osteotomy (PAO) for activities other than walking.

PURPOSE

Assess patient-reported outcomes, hip strength, lower extremity joint moments, and moment distributions during sit-to-stand in a female athlete during the one-year period following PAO.

METHODS

Patient-reported outcomes (i.e. hip and groin outcome score; HAGOS), hip strength, and lower extremity joint moments and contributions to peak support moment during the sit-to-stand were assessed pre-operatively and at 6 weeks, 12 weeks, 6 months, and 13 months following PAO.

RESULTS

HAGOS scores returned to normative ranges, however overall quality of life scores remained low. By 13-month muscle strength recovered to baseline values. Following surgery, peak support and hip extensor moments were higher on the non-surgical limb, but the contributions of hip, knee, and ankle to peak support moment did not change. On the surgical limb, while peak support moments were lower, the hip contributed less, and the knee contributed more. These asymmetric loading patterns gradually normalized over 1 yea-.

CONCLUSIONS

Asymmetric loading following PAO should be considered when developing rehabilitation plans following this surgery.

The Femoral Head "Divot" Sign: A Useful Arthroscopic Sign of Hip Microinstability

Background:

A femoral head “divot” is a rare finding during hip arthroscopy. A linear chondral indentation can be observed on the femoral head, just lateral and parallel to the acetabular labrum.

Purpose/Hypothesis:

The purpose of this study was to describe a novel arthroscopic sign and retrospectively review patients with this finding. We hypothesized that this sign would be found in patients with characteristics consistent with hip microinstability.

Study Design:

Case series; Level of evidence, 4.

Methods:

Intraoperative images of patients undergoing primary hip arthroscopy between July 2017 and July 2019 were reviewed for evidence of a femoral head divot. Preoperative characteristics, physical examination findings, radiographic measurements, and magnetic resonance imaging (MRI) findings were described.

Results:

Of 690 available cases, 14 cases (13 patients; 2.0%) had evidence of a femoral head divot. The mean patient age was 29.1 years, and all but 1 patient (92.3%) were female. Ligamentous laxity was present in 81.8% of patients, anterior apprehension test was positive in 78.6%, and painful internal snapping was present in 50.0%. The mean lateral center-edge angle, anterior center-edge angle, and Tönnis angle were 19.2°, 20.3°, and 12.4°, respectively. The divot was identified in 5 of 12 available MRI scans, most commonly on axial proton density sequence. Intraoperatively, all hips had labral tears, iliopsoas bursitis was demonstrated in 78.6%, and the ligamentum teres was damaged in 42.9%. Labral repair was performed in 12 of the 14 hips, with 2 patients undergoing labral reconstruction. Iliopsoas fractional lengthening was performed in 50.0%, capsular plication was performed in 78.6%, and capsular repair was performed in the remainder.

Conclusion:

The femoral head divot sign is a rare arthroscopic finding during hip arthroscopy. The results of this study demonstrated that patients who have a divot also present with characteristic radiographic or physical examination findings of hip microinstability due to either acetabular dysplasia or ligamentous laxity. Recognition of a femoral head divot may be valuable for the diagnosis of microinstability during hip arthroscopy and may help guide appropriate management, such as capsular plication. Further studies are needed to determine the exact prevalence of the femoral head divot in patients with microinstability and to evaluate the effect of this finding on patient outcomes.

Subject-specific geometry affects acetabular contact pressure during gait more than subject-specific loading patterns

Finite element modeling (FEM) can predict hip cartilage contact mechanics. This study investigated how subject-specific boundary conditions and joint geometry affect acetabular cartilage contact mechanics using a multi-scale workflow. For two healthy subjects, musculoskeletal models calculated subject-specific hip kinematics and loading, which were used as boundary conditions for FEM. Cartilage contact mechanics were predicted using either generic or subject-specific FEM and boundary conditions. A subject-specific mesh resulted in a more lateral contact. Effects of subject-specific boundary conditions varied between both subjects. Results highlight the complex interplay between loading and kinematics and their effect on cartilage contact mechanics.

Patient Age and Hip Morphology Alter Joint Mechanics in Computational Models of Patients With Hip Dysplasia

BACKGROUND

Older patients (> 30 years) undergoing periacetabular osteotomy (PAO) to delay THA often have inferior patient-reported outcomes than younger adult patients (< 30 years). It is unclear how patient age affects hip morphology, mechanics, or patient-reported outcome scores.

QUESTIONS/PURPOSES

(1) Is increased patient age associated with computationally derived elevations in joint contact stresses? (2) Does hip shape affect computationally derived joint contact stresses? (3) Do computationally derived joint contact stresses correlate with visual analog scale (VAS) pain scores evaluated at rest in the clinic at a minimum of 1 year after surgery?

METHODS

A minimum of 1 year of clinical followup was required for inclusion. The first 15 patients younger than 30 years of age, and the first 15 patients older than 30 years of age, who underwent PAO for treatment of classic dysplasia (lateral center-edge angle < 25°) who met the minimum followup were selected from a historical database of patients treated by a single surgeon between April 2003 and April 2010. The older cohort consisted of 14 females and one male with a median age of 41 years (range, 31-54 years). The younger cohort consisted of 10 females and five males with a median age of 19 years (range, 12-29 years). Median followup for the older than 30 years versus younger than 30 years cohort was 19 months (range, 12-37 months) versus 24 months (range, 13-38 months). Pre- and postoperative hip models were created from CT scans for discrete element analysis (DEA) contact stress computations. DEA treats contacting articular surfaces as rigid bodies (bones) separated by a bed of compressive springs (cartilage), the deformation of which governs computation of joint contact stresses. This technique greatly simplifies computational complexity compared with other modeling techniques, which permits patient-specific modeling of larger cohorts. Articular surface shape was assessed by total root mean square deviation of each patient's acetabular and femoral cartilage geometry from sphericity. Preoperative and postoperative VAS pain scores evaluated at rest in the clinic were correlated with computed contact stresses.

RESULTS

Patients older than 30 years had higher predicted median peak contact stress preoperatively (13 MPa [range, 9-23 MPa; 95% confidence interval {CI}, 11-15 MPa] versus 7 MPa [range, 6-14 MPa; 95% CI, 6-8 MPa], p < 0.001) but not postoperatively (10 MPa [range, 6-18 MPa; 95% CI, 8-12 MPa] versus 8 MPa [range, 6-13 MPa; 95% CI, 7-9 MPa], p = 0.137). Deviation from acetabular sphericity positively correlated with preoperative peak contact stress (R = 0.326, p = 0.002) and was greater in the older cohort (0.9 mm [range, 0.8-1.5 mm; 95% CI, 0.8-1.0 mm] versus 0.8 mm [range, 0.6-0.9 mm; 95% CI, 0.7-0.9 mm], p = 0.014). Peak preoperative contact stress did not correlate with preoperative VAS pain score (R = 0.072, p = 0.229), and no correlation was found between change in peak contact stress and change in VAS score (R = 0.019, p = 0.280).

CONCLUSIONS

Patients over the age of 30 years with dysplasia had less spherical acetabula and higher predicted preoperative contact stress than those younger than 30 years of age. Future studies with larger numbers of patients and longer term functional outcomes will be needed to determine the role of altered mechanics in the long-term success of PAO varying with patient age.

CLINICAL RELEVANCE

These findings suggest that long-term exposure to abnormal joint loading may have deleterious effects on the hip geometry and may render the joint less amenable to joint preservation procedures. Given the lack of a direct relationship between mechanics and pain, orthopaedic surgeons should be particularly critical when evaluating three-dimensional dysplastic hip morphology in patients older than 30 years of age to ensure beneficial joint reorientation.