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.

Isolated changes in femoral version do not alter intra-articular contact mechanics in cadaveric hips.

Abnormal femoral version is a deformity in the angle between the femoral neck and the transcondylar axis of the knee. Both femoral anteversion and retroversion alter passive and active rotation of the hip and are associated with intra-articular or extra-articular impingement. However, little is known about the effect of abnormal femoral version on intra-articular hip contact stresses. To quantify the effect of femoral version on hip contact stress, five cadaveric pelvis specimens were mechanically tested with a hip-specific Tekscan sensor inserted in the joint space. Specimens were oriented in a heel-strike position and loaded with 1000 N of compressive force. Pressure measurements were recorded by the Tekscan sensor with the femur oriented in 0°, 15°, and 30° of version. At the completion of testing, specimens were locked into place at 0° and post-test CT scans were obtained to register the pressure sensor measurements to the joint anatomy. There were minor changes in contact area (<7%) and translation of the peak contact stress location (8.8 ± 7.6 mm). There was no significant change in peak contact stress (p = 0.901) in either the retroverted (0°) or anteverted (30°) conditions relative to normal version (15°) under identical gait-related loading conditions. While abnormalities in patient gait and resultant joint loading caused by femoral version abnormalities may contribute to hip pain, the present findings would suggest that future joint degeneration in hips with version abnormalities are not simply the result of abnormal contact stress induced by joint incongruity due to femoral version abnormalities.

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.

Unaddressed Cam Deformity Is Associated with Elevated Joint Contact Stress After Periacetabular Osteotomy.

BACKGROUND: Femoral cam deformity is frequently present in patients with acetabular dysplasia. Computational modeling can be used to identify how this deformity affects joint mechanics. Our purpose was to identify the relationship between cam deformity and joint contact stress after periacetabular osteotomy (PAO). We hypothesized that cam deformity is associated with an increase in peak joint contact stress after PAO. METHODS: This was a retrospective review of patients treated for hip dysplasia with PAO without femoral osteochondroplasty. Patient-specific hip models created from preoperative and postoperative computed tomography (CT) scans were evaluated using discrete element analysis to determine maximum joint contact stress after PAO. Twenty hips with a postoperative increase in maximum contact stress were compared with 20 that demonstrated decreased maximum contact stress. Hips were assessed for cam deformity on cross-sectional imaging. Radiographic measures of acetabular dysplasia before and after PAO were assessed and compared with the change in maximum contact stress after PAO. RESULTS: There was a moderate relationship between the change in maximum contact stress and the α angle (r = 0.31; p = 0.04), and the average α angle in the hips with increased maximum contact stress was significantly different from that in the hips with decreased joint contact stress (51° ± 11.4° versus 42° ± 5.1°; p = 0.04). All 6 hips with an α angle of >60° demonstrated increased joint contact stress. CONCLUSIONS: Cam deformity is common in patients with hip dysplasia. In our study, α angles of >60° were associated with increased postoperative joint contact stress. The α angle should be assessed preoperatively, and deformity should be addressed for optimal joint mechanics after PAO. CLINICAL RELEVANCE: A reduction in joint contact stress is a proposed mechanism for the increased joint longevity following periacetabular osteotomy for hip dysplasia. Impingement from abnormal femoral offset negatively impacts clinical outcome, but this finding has not been evaluated from a biomechanical perspective previously and a threshold for performing femoral osteochondroplasty has not been established previously. This study provides biomechanical evidence supporting surgical management of femoral cam deformity for an α angle of >60°.

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

Gait modifications in acetabular dysplasia patients may influence cartilage contact stress patterns within the hip joint, with serious implications for clinical outcomes and the risk of developing osteoarthritis. The objective of this study was to understand how the gait pattern used to load computational models of dysplastic hips influences computed joint mechanics. Three-dimensional pre- and post-operative hip models of thirty patients previously treated for hip dysplasia with periacetabular osteotomy (PAO) were developed for performing discrete element analysis (DEA). Using DEA, contact stress patterns were calculated for each pre- and post-operative hip model when loaded with an instrumented total hip, a dysplastic, a matched control, and a normal gait pattern. DEA models loaded with the dysplastic and matched control gait patterns had significantly higher (p = 0.012 and p < 0.001) average pre-operative maximum contact stress than models loaded with the normal gait. Models loaded with the dysplastic and matched control gait patterns had nearly significantly higher (p = 0.051) and significantly higher (p = 0.008) average pre-operative contact stress, respectively, than models loaded with the instrumented hip gait. Following PAO, the average maximum contact stress for DEA models loaded with the dysplastic and matched control patterns decreased, which was significantly different (p < 0.001) from observed increases in maximum contact stress calculated when utilizing the instrumented hip and normal gait patterns. The correlation between change in DEA-computed maximum contact stress and the change in radiographic measurements of lateral center-edge angle were greatest (R2 = 0.330) when utilizing the dysplastic gait pattern. These results indicate that utilizing a dysplastic gait pattern to load DEA models may be a crucial element to capturing contact stress patterns most representative of this patient population.

Discrete element analysis is a valid method for computing joint contact stress in the hip before and after acetabular fracture.

Evaluation of abnormalities in joint contact stress that develop after inaccurate reduction of an acetabular fracture may provide a potential means for predicting the risk of developing post-traumatic osteoarthritis. Discrete element analysis (DEA) is a computational technique for calculating intra-articular contact stress distributions in a fraction of the time required to obtain the same information using the more commonly employed finite element analysis technique. The goal of this work was to validate the accuracy of DEA-computed contact stress against physical measurements of contact stress made in cadaveric hips using Tekscan sensors. Four static loading tests in a variety of poses from heel-strike to toe-off were performed in two different cadaveric hip specimens with the acetabulum intact and again with an intentionally malreduced posterior wall acetabular fracture. DEA-computed contact stress was compared on a point-by-point basis to stress measured from the physical experiments. There was good agreement between computed and measured contact stress over the entire contact area (correlation coefficients ranged from 0.88 to 0.99). DEA-computed peak contact stress was within an average of 0.5 MPa (range 0.2-0.8 MPa) of the Tekscan peak stress for intact hips, and within an average of 0.6 MPa (range 0-1.6 MPa) for fractured cases. DEA-computed contact areas were within an average of 33% of the Tekscan-measured areas (range: 1.4-60%). These results indicate that the DEA methodology is a valid method for accurately estimating contact stress in both intact and fractured hips.

Diagnosis and Management of Borderline Hip Dysplasia and Acetabular Retroversion.

Borderline hip dysplasia and acetabular retroversion are common radiographic findings in young individuals with and without hip pain. Orthopaedic surgeons should be knowledgeable about the radiographic findings, diagnosis, and appropriate nonsurgical and surgical treatment of these conditions. Borderline hip dysplasia is generally defined by a lateral center edge angle of Wiberg from 20 to 25° (some define as 18-25°) and is a cause of joint microinstability. The degree of soft tissue laxity can have significant implications for joint stability in patients with borderline hip dysplasia. The most common presenting symptoms are groin pain and lateral hip pain. Acetabular retroversion is defined by radiographic findings of crossover sign, ischial spine sign, and posterior wall sign. Individuals with symptomatic retroversion have a clinical presentation consistent with impingement, groin pain with flexion activities, and less commonly lateral hip pain. Physical therapy has been shown to improve symptoms in a subset of individuals with these conditions. There are multiple recent publications about arthroscopic treatment of patients with borderline hip dysplasia. These reports generally find that good short-term outcomes can be expected when using arthroscopic techniques that include labral preservation/repair and capsular plication. There are limited reports of periacetabular osteotomy as a treatment for borderline hip dysplasia. Publications focusing specifically on surgical treatment of acetabular retroversion are also infrequent. Periacetabular osteotomy has been shown to have superior long-term clinical outcomes to surgical hip dislocation with anterior rim trimming in patients with all three radiographic findings of retroversion. Arthroscopic treatment has been shown to have good short-term outcomes. Future work in the areas of borderline hip dysplasia and acetabular retroversion should focus on reporting long-term clinical follow-up of these surgical treatments and using computation techniques as a tool to determine appropriate surgical and nonsurgical treatment for each individual patient.