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. 2019 Feb 6;477(5):1235–1245. doi: 10.1097/CORR.0000000000000621

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

Holly D Thomas-Aitken 1,2, Jessica E Goetz 1,2, Kevin N Dibbern 1,2, Robert W Westermann 1,2, Michael C Willey 1,2, Timothy S Brown 1,2,✉
PMCID: PMC6494307  PMID: 30801275

Abstract

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 (R2 = 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 (R2 = 0.072, p = 0.229), and no correlation was found between change in peak contact stress and change in VAS score (R2 = 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.

Introduction

Acetabular dysplasia, typically characterized by the presence of a shallow acetabulum and inadequate femoral head coverage, alters the transfer of forces through the hip [29]. Altered hip biomechanics subject the hip to elevated contact stresses, which often result in hip pain and accelerated progression of osteoarthritis (OA) [10, 16, 23]. The most common surgical treatment for acetabular dysplasia in the adolescent and young adult is periacetabular osteotomy (PAO), which permits multiplanar acetabular reorientation for hip stabilization and reduction of contact stress and pain [13, 35]. PAO has been shown to provide good to excellent results in 73% to 81% of patients after followup times ranging from 10 to 20 years [35, 38, 48], and PAO may decrease the risk of OA progression [13, 47].

As summarized in a systematic review, most patients undergoing PAO for acetabular dysplasia are adolescents and young adults with the typical age at the time of operation between 21 and 37 years [11]. Many studies have found decreased survivorship of PAO in patients older than 40 years of the age [14, 19, 26, 31, 38, 45]. Although older age has been associated with development of OA and conversion to THA after PAO, specific mechanistic factors that could be contributing to poor patient-reported outcomes in this patient population such as age-related differences in joint morphology and/or joint contact stress have not yet been well defined. Numerous studies have utilized shape models to describe variations in hip morphology [4, 5, 15]. However, these shape models are based on two-dimensional imaging and do not quantitatively assess the degree of hip morphologic variations. Computational models based on three-dimensional techniques and root mean square curvature have shown less spherical shapes present in dysplastic hips [17, 28] but did not evaluate the relationship between such morphologic differences and the resulting joint mechanics. An accurate understanding of how hip shape alters joint contact stresses may provide insight into contributing factors that lead to decreased PAO survivorship in patients older than 30 years of age.

To identify potential reasons underlying the negative effects of age on patient-reported and survivorship outcomes after PAO, we asked: (1) Is increased patient age associated with elevations computationally derived 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?

Patients and Methods

Under institutional review board approval, patients who underwent PAO for treatment of symptomatic acetabular dysplasia by a single orthopaedic surgeon (TOM) at our institution between 2003 and 2010 were retrospectively reviewed. Patients were eligible for inclusion if they were indicated to have borderline (20° ≤ lateral center-edge angle [LCEA] ≤ 25°) or classic dysplasia (LCEA < 20°) preoperatively. Patients with acetabular retroversion or dysplasia secondary to Perthes disease were excluded. During the 8-year study period, 115 patients who met these criteria underwent surgery and were eligible for inclusion. However, 54 patients were lost to followup before 1 year, which was the minimum for inclusion, leaving 61 patients potentially eligible. Of these 61 patients, the first 15 patients younger than 30 years of age and the first 15 patients older than 30 years of age who met all of the inclusion criteria were included in the study. To determine if the resulting cohorts were equivalent after our selection process, two fellowship-trained hip surgeons (TSB, RWW) graded arthritis based on the Tönnis classification [43], and a third fellowship-trained hip surgeon (MCW) measured LCEA, Tönnis angle, and extrusion index on preoperative radiographs. There was no difference in the median Tönnis grade between patients younger than or older than 30 years of age (1 [range, 0-1.5] versus 1 [range, 0.5-1.5], p = 0.302). The median preoperative radiographic measurement of LCEA was not different between patients younger than 30 years (20° [range, 2°-25°]) and older than 30 years (12° [range, 6°-25°], p = 0.081). However, there were differences in median preoperative radiographic measurements of Tönnis angle (younger: 11° [range, 4°-38°] versus older: 19° [range, 8°-28°], p = 0.014) and extrusion index (younger: 23% [range, 16%-43%] versus older: 33% [range, 20%-46%], p = 0.025) between age groups.

In the patients younger than 30 years of age, median age at the time of operation was 19 years (range, 12-29 years), and the median body mass index (BMI) was 25 kg/m2 (range, 22-41 kg/m2). In the patients older than 30 years of age, median age was 41 years (range, 31-54 years), and the median BMI was 32 kg/m2 (range, 21-45 kg/m2). There was no difference (p = 0.479) in median followup time between patients younger than 30 years of age (24 months [range, 13-38 months]) and patients older than 30 years of age (19 months [range, 12-37 months]). Preoperatively and postoperatively at latest followup, VAS pain scores evaluated at rest in the clinic were obtained by chart review.

Discrete element analysis (DEA) was implemented to compute cartilage contact stresses for each patient’s hip. DEA is a modeling technique with greatly simplified computational complexity compared with other computational modeling techniques such as finite element analysis (FEA). In this technique, contacting surfaces are represented as rigid bodies separated by a bed of compressive springs [32]. When a load is applied to one or all of the rigid bodies in the model, the resulting spring deformation is used to determine the contact stresses between the bodies. The reduced modeling complexity and rapid computational time of DEA permit evaluation of larger numbers of patient-specific models than FEA studies without hindering accuracy of the resulting stress calculations [2].

To compute cartilage contact stresses for each patient-specific hip model using DEA, preoperative and postoperative clinical CT scans that had been acquired for each patient (Fig. 1) were segmented through use of a semiautomated, watershed-based algorithm previously developed in MATLAB (Mathworks, Natick, MA, USA) [41]. Minor errors in edge definition or holes in cancellous bone were manually corrected. Interobserver and intraobserver variability associated with this manual correction step of segmentation was assessed. Comparison among three different segmenters (HDT-A, MA, KS) over 3 nonconsecutive days resulted in a median absolute difference of 0.8 MPa (range, 0-2.1 MPa) in the computed peak contact stress and 0.1 MPa (range, 0-0.2 MPa) in the computed mean contact stress. Variation in segmentation by the same segmenter over 3 nonconsecutive days resulted in a median absolute difference of 1.2 MPa (range, 0.3-2.1 MPa) in the computed peak contact stress and 0.1 MPa (range, 0-0.2 MPa) in the computed mean contact stress [40]. These results indicate a minimal effect of manual segmentation on the resulting contact stress calculation.

Fig. 1 A-B.

Fig. 1 A-B

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(A) In this illustrative example of a nonweightbearing coronal CT scan of a 17-year-old patient with hip dysplasia, the acetabulum and femoral head are slightly flattened but have mostly spherical articular surfaces. (B) In this illustrative example of a nonweightbearing CT scan of a 45-year-old patient with hip dysplasia, there is a noticeable deformity of the acetabulum (indicated by arrow) that results in calculation of an aspherical articular surface.

Triangulated surface models of each patient’s segmented femoral, pelvic, and spinal bony geometry were then imported into Geomagic® Design X (3D Systems, Inc, Rock Hill, SC, USA) where they were smoothed to eliminate stairstep artefact. To approximate the articular cartilage surfaces, the femoral and acetabular subchondral bone surfaces were first projected a uniform distance of 1 mm. A custom iterative smoothing algorithm was then applied to the projected surfaces to generate a realistic, smoothly continuous articular surface with nonuniform cartilage thickness [34]. This method of cartilage generation does not permit inclusion of a labrum; however, peak cartilage contact stresses calculated utilizing this modeling methodology have been found to be within a mean of 0.5 MPa (range, 0.2-0.8 MPa) of actual peak contact stresses measured using Tekscan sensors in cadaveric hips, and whole joint stress distributions show excellent agreement with Tekscan-measured contact stress distributions over the entire contact area (R2 = 0.93-0.99) [44]. Cartilage was assigned isotropic linear-elastic material properties (E = 8 MPa, ν = 0.42) [44].

Anatomic landmarks were identified on each patient-specific model and used to align each model to the hip coordinate system defined by Bergmann et al. [9]. Models were loaded by applying an average series of joint reaction forces and hip rotations obtained from 32 patients with hip dysplasia through three-dimensional motion capture and musculoskeletal modeling [36, 42]. Those forces and rotations were discretized into 13 evenly distributed loading cases representative of the stance phase of walking gait. All applied forces were scaled according to each patient’s body mass. DEA was performed using a custom Newton’s method solver previously developed in MATLAB [22] to compute contact stress distributions during the applied gait cycle loading. To evaluate whether any differences in computed contact stress between age groups were caused solely by scaling the applied forces according to patient body mass, all DEA models were also analyzed using the mean weight of all 30 patients (79.4 kg) as the scaling factor.

Preoperative and postoperative peak contact stresses computed at each of the 13 time points were compared between patient age groups. Preoperative and postoperative VAS pain scores obtained by chart review were correlated with DEA-computed contact stresses. To quantify the level of acetabular deformity, a sphere was fit to each patient’s preoperative acetabular cartilage geometry using a custom MATLAB program (Fig. 2). The difference between the radius of the fitted sphere and the actual distance from the sphere center to each vertex of the acetabular cartilage surface was calculated. This difference was considered to be the error from sphericity at that vertex location. These errors were then used to compute the total root mean square deviation (RMSD) from acetabular sphericity using the equation:

graphic file with name abjs-477-1235-g002.jpg

where ei = the error between the sphere radius and the distance from the sphere center to vertex i and n = the total number of acetabular cartilage vertices. A RMSD value of zero indicates a perfectly spherical cartilage surface, and increasing RMSD values indicate increasingly aspherical acetabular shapes. This process was repeated for each patient’s preoperative femoral head cartilage geometry to assess the level of femoral head deformity.

Fig. 2.

Fig. 2

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The sphere-fitting technique used to assess the level of deformity is illustrated for the acetabular surface. A best-fit sphere (blue) is found for the patient-specific cartilage geometry (black dots). Arrows indicate cartilage locations that deviate substantially from sphericity. The error between the radius of this sphere and the actual distance from the sphere center to each cartilage vertex was calculated. These errors were then used to compute the total RMSD from acetabular sphericity. The same process was used with the patient-specific femoral head cartilage surfaces to determine femoral head sphericity.

Statistical Analysis

Two-sided Wilcoxon rank-sum tests performed in SAS 9.4 (SAS Institute Inc, Cary, NC, USA) were used to compare differences in computed contact stress results between patient age groups. Linear regression analysis with Spearman correlation coefficients was used to evaluate univariate relationships among contact stress, patient age, and acetabular and femoral head sphericity. Significance was set at p ≤ 0.05.

Results

The preoperative peak contact stress was higher for patients older than 30 years (median 13 MPa [range, 9-23 MPa; 95% confidence interval {CI}, 11-15 MPa]) versus those younger than 30 years (median 7 MPa [range, 6-14 MPa; 95% CI, 6-8 MPa], p < 0.001; Fig. 3). After PAO, peak contact stress in patients older than 30 years decreased (median difference: 3 MPa, p = 0.039; median 10 MPa [range, 6-18 MPa; 95% CI, 8-12 MPa]), but not for patients younger than 30 years of age (8 MPa [range, 6-13 MPa; 95% CI, 7-9 MPa], p = 0.389). There was no difference in median peak contact stress after PAO between cohorts (median 10 MPa [range, 6-18 MPa; 95% CI, 8-12 MPa] versus median 8 MPa [range, 6-13 MPa; 95% CI, 7-9 MPa], p = 0.167; Fig. 4).

Fig. 3.

Fig. 3

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Preoperative computed peak contact stress is higher (*p < 0.05) in patients > 30 years of age than in patients < 30 years of age. Peak contact stress in patients > 30 years of age decreases (#p < 0.05) preoperatively to postoperatively. However, postoperative peak contact stress is still higher (†p < 0.05) in patients > 30 years of age than in patients < 30 years of age at time points 7 and 13. Dots indicate median values of peak contact stress, and error bars indicate the interquartile range of the median values for each of the 13 time points.

Fig. 4.

Fig. 4

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Preoperative and postoperative computed contact stress distributions are overlaid on the corresponding hip models for two patients with hip dysplasia. The dark blue color indicates no contact between the acetabular and femoral cartilage surfaces at that location. The left column shows a hip model for a patient aged < 30 years at the time of PAO. The right column shows a hip model for a patient aged > 30 years at the time of PAO. The patient aged > 30 years had a much higher preoperative contact stress than the younger patient. PAO decreased the peak contact stress in the older patient, but this decreased stress was still higher than the peak contact stress in the younger patient.

The RMSD from acetabular sphericity was greater for patients older than 30 years of age, indicating a less spherical acetabular shape than in patients younger than 30 years of age (median 0.9 mm [range, 0.6-0.9 mm; 95% CI, 0.7-0.9 mm] versus 0.8 mm [range, 0.8-1.5 mm; 95% CI, 0.8-1.0 mm], p = 0.014). There was a weakly positive linear correlation between patient age and RMSD from acetabular sphericity (R2 = 0.228, p = 0.021; Fig. 5A). There was also a slight positive correlation between RMSD from acetabular sphericity and preoperative peak contact stress (R2 = 0.362, p = 0.002; Fig. 5B). On the opposite side of the joint, there was no difference in median RMSD from femoral head sphericity between patients older than and/or younger than 30 years of age (1.1 mm [range, 0.6-1.7 mm] versus 1.0 mm [range. 0.5-1.9 mm], p = 0.967). No correlations were found between patient age and RMSD from femoral head sphericity (R2 = 0.001, p = 0.840) or between RMSD from femoral head sphericity and preoperative peak contact stress (R2 = 0.076, p = 0.129).

Fig. 5 A-B.

Fig. 5 A-B

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(A) Patient age correlates with RMSD from acetabular sphericity (R2 = 0.228, p = 0.021) but not with RMSD from femoral head sphericity (R2 = 0.001, p = 0.840). (B) Preoperative peak computed contact stress correlates with RMSD from acetabular sphericity (R2 = 0.362, p = 0.002) but not with RMSD from femoral head sphericity (R2 = 0.076, p = 0.129).

Peak preoperative contact stress did not correlate with preoperative VAS pain score across all 30 patients (R2 = 0.072, p = 0.229; Fig. 6A). Although 13 of 15 (87%) patients older than 30 years of age had a decrease in both VAS pain and peak contact stress postoperatively, no correlation was found between the magnitude of change in peak contact stress and change in VAS score (R2 = 0.019, p = 0.280; Fig. 6B).

Fig. 6 A-B.

Fig. 6 A-B

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(A) Preoperative computed peak contact stress does not correlate with preoperative VAS pain score (R2 = 0.072, p = 0.018). (B) The change in peak contact stress preoperatively to postoperatively does not correlate with the change in VAS pain score preoperatively to postoperatively (R2 = 0.019, p = 0.009).

Other Findings

There was no correlation between patient age and patient weight (R2 = 0.063, p = 0.204); however, utilizing the mean patient weight as the scaling factor for the applied forces altered the computed peak contact stresses (Table 1), indicating that patient weight does play a role in the resulting contact stress. These results indicate that there are differences in contact stress associated with patient age, patient weight, and acetabular morphology at the time of PAO.

Table 1.

The comparative computed peak contact stress data for hip models of both patient age groups with forces scaled by patient-specific and mean weight

graphic file with name abjs-477-1235-g008.jpg

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Discussion

The goal of PAO for treatment of acetabular dysplasia is to reorient the acetabulum to improve joint stability, reduce pain, and prevent OA development in the adolescent and young adult population. Although older patients can undergo PAO to delay THA, advanced age at the time of operation has been associated with poor and unpredictable patient-reported outcomes [14, 19, 26, 31, 38, 45]. In this study, patients older than age 30 years who underwent PAO for acetabular dysplasia were found to have less spherical acetabula and higher computed preoperative contact stress, which was frequently associated with higher VAS pain scores before surgery, in comparison with patients who underwent PAO before the age of 30 years.

This work has several limitations that warrant discussion. The sample size of 15 patients per age group is small; therefore, statistically significant/insignificant findings should be interpreted with caution. With the greater proportion of women in this study and the small number of patients, we were unable to assess differences in contact stress patterns between sexes. Although there were no differences in Tönnis grade or LCEA between age groups, the group of patients older than 30 years of age did have a higher median Tönnis angle and extrusion index than the group of patients younger than 30 years of age. These differences in acetabular slope and coverage highlight the fact that patients older than 30 years have more aspherical acetabular shapes. VAS pain scores, especially as obtained from a retrospective chart review, is a relatively insensitive metric of patient outcome. Because these scores may vary with activity or individual perception of pain, use of functional outcome metrics would have been preferable, but they were less reliably available in this historical cohort.

Clinical indication for PAO in this patient series was LCEA < 20° with hip pain that was not significantly improved with physical therapy and/or other conservative treatments, including activity modification, intraarticular corticosteroid injections, or antiinflammatory medications. Patients with hypermobility and LCEA < 25° were indicated for PAO after failing conservative treatment. Patients with Perthes disease and retroversion were not included in this patient series. These indications did not change based on the age of the patient. We had concern that older patients would have a higher incidence of OA, but this was not observed in our series because there was no difference between age groups (median Tönnis grade 1 [range, 0-1.5] versus 1 [range, 0.5-1.5], p = 0.302).

There are also several limitations specific to the modeling process. DEA-computed cartilage contact stress was the main outcome measure reported, because contact stress has been frequently assessed in computational hip studies [1, 6, 44] and associated with increased OA risk [16, 25]. Cartilage shear stresses are also known to contribute to OA development [37]; however, the DEA methodology and static loading positions used to simulate walking gait in this work made shear negligible from a modeling standpoint. Cartilage was assumed to be an isotropic, linearly elastic material, which greatly simplifies its recognized rate-dependent, poroelastic behavior [18, 27]. However, the relatively high cartilage loading rate that occurs during walking [7, 21] limits fluid exudation, supporting the use of a linearly elastic material property definition. The cartilage generation methodology used in this work precludes the inclusion of a labrum and produces thinner cartilage surfaces (0.8 ± 0.2 mm for the femoral head and 1.1 ± 0.3 mm for the acetabulum) than previous studies in the hip [3, 8, 33], yet this methodology has been shown to produce valid contact stress predictions within 0.5 MPa of experimentally measured stresses obtained with Tekscan sensors in cadaveric hips [44]. All of the patient-specific models were aligned to the coordinate system defined by Bergmann et al. [9] to facilitate walking gait loading. By ensuring constant orientation between models, we were unable to account for patient-specific factors such as pelvic tilt. The gait pattern used to load both the preoperative and postoperative DEA models was the same average from 32 patients with hip dysplasia [36], which may not necessarily illustrate how any particular patient with hip dysplasia would walk. Additionally, the altered gait in these individuals may or may not persist after PAO [12, 20, 30, 39, 46], the interhip distance may change [24], and altered muscle moment arms may alter a patient’s postoperative joint reaction forces. However, given that no gait data were collected for the 30 patients modeled in this study, loading their DEA models with an average gait pattern collected from individuals with dysplastic deformities was the best available option.

Another limitation of this study is that there is limited clinical followup for this retrospective series of patients. We included patients with a minimum of 1-year clinical followup; however, most of these patients were only evaluated postoperatively with VAS pain scores at rest in the clinic. Other patient-reported outcomes that evaluate patient function (for example, WOMAC) were not available for this retrospective cohort. To provide a more complete assessment of the relationship between long-term patient outcomes and computed contact stress, future longitudinal studies will be required.

In this study, patients older than 30 years were found to have higher DEA-computed preoperative contact stress than patients younger than 30 years despite no difference in preoperative Tönnis grade. This is important, because most surgeons are limited to preoperative radiographs for determination of cartilage health before indication for PAO, which our study suggests may be insufficient in patients older than 30 years. Numerous studies have associated older age with decreased PAO survivorship [14, 38, 45] and higher risk of conversion to THA [26, 31, 38]. A review of 23 studies found that patients with successful patient-reported outcomes after PAO had a mean age of 28.7 years at the time of operation compared with a mean age of 30.9 years for patients who had subsequent conversion to THA [31]. Patients older than 40 years who undergo PAO have been found to have worse (p < 0.005) WOMAC pain and function scores after 4 years than patients of the same age range undergoing THA [14] with 24% needing THA within an average of 8.4 years [26]. Computational mechanics studies such as those by Abraham et al. [1] and Armiger et al. [6] have found that PAO reduces contact stresses computed in patient cohorts aged 29.8 ± 5.8 years and 35 years (range, 20-50 years), respectively. However, although these studies supported improved contact mechanics after PAO, they did not separately evaluate how patient age at the time of operation affects the contact stress distributions. To the best of our knowledge, this is the first study that has directly investigated how patient age at the time of PAO relates to the contact mechanics of the dysplastic hip.

Interestingly, although older patients with dysplasia were found to have less spherical acetabula, there were no differences found in femoral head sphericity between patient age groups. Ahedi et al. [5] reported the amount of acetabular coverage and femoral head and neck shape were associated with the incidence and progression of hip OA, whereas a statistical shape modeling study by Agricola et al. [4] found five shape variants that were associated with THA but could find no shape associations with OA. The lack of relationship between femoral head shape and changes in joint mechanics in our study would support no relationship with OA. Incorporating the acetabular side of the joint, Henak et al. [17] calculated the root mean square curvature between the articular surfaces of computational hip models to assess joint congruence. Although no differences in local joint congruence were found between dysplastic and normal hips, they too found less spherical hip shapes in patients with dysplasia [17]. Taken together with our findings, these studies suggest a loss of congruency in dysplastic hips that is occurring primarily on the acetabular side, becoming more pronounced in older patients whose hips have had longer term exposure to altered loading that drives bony remodeling.

Although higher preoperative contact stresses in patients older than 30 years of age were frequently observed with higher preoperative VAS pain scores evaluated at rest in the clinic, we did not find a correlation between the two measures. Similarly, whereas 13 of 15 (87%) patients older than 30 years of age had a decrease in both VAS pain and peak contact stress postoperatively, no correlation was found between the amount of change in contact stress and the amount of change in pain score. There was no specific amount of contact stress magnitude or reduction that resulted in a specific reduction in VAS pain. It is possible that the subjective nature of the VAS pain metric makes it difficult to establish a direct relationship with the objective metric of computed contact stress. More objective patient-reported outcomes such as WOMAC and Harris hip scores may have a stronger relationship with biomechanical data. Future computational studies comparing predicted contact stresses in patients with longitudinal patient-reported outcome scores will be important in understanding how a patient’s pain and function are affected by the altered mechanics after PAO.

Patients with hip dysplasia aged older than 30 years at the time of PAO were found to have less spherical acetabula and higher preoperative computed contact stress, which may indicate that long-term exposure of their hips to damaging loads produced locations of high contact stress that resulted in bony remodeling. This phenomenon may not only apply to patients with hip dysplasia, but also to older individuals without specific hip pathology [24]. A patient’s age and hip shape are important factors contributing to the resulting hip mechanics, although the interplay between those mechanical factors and patient-reported outcomes of pain are unclear. Recognition of these geometric factors may assist in preoperative counseling and planning to ensure that the planned acetabular reorientation reduces the contact stress on the hip.

Acknowledgments

We thank Todd O. McKinley MD, for providing the patient cohort for this study and Megan Andrew and Kara Schneider for assisting with image segmentation.

Footnotes

One or more of the authors (HDT-A, JEG, MCW) received grants from the Orthopaedic Research and Education Foundation.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA-approval status, of any drug or device prior to clinical use.

Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

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