Finite Element Analysis Examining the Effects of Cam FAI on Hip Joint Mechanical Loading Using Subject-Specific Geometries During Standing and Maximum Squat

Background:

Cam femoroacetabular impingement (FAI) can impose elevated mechanical loading in the hip, potentially leading to an eventual mechanical failure of the joint. Since in vivo data on the pathomechanisms of FAI are limited, it is still unclear how this deformity leads to osteoarthritis.

Purpose:

The purpose of this study was to examine the effects of cam FAI on hip joint mechanical loading using finite element analysis, by incorporating subject-specific geometries, kinematics, and kinetics.

Questions:

The research objectives were to address and determine: (1) if hips with cam FAI demonstrate higher maximum shear stresses, in comparison with control hips; (2) the magnitude of the peak maximum shear stresses; and (3) the locations of the peak maximum shear stresses.

Methods:

Using finite element analysis, two patient models were control-matched and simulated during quasi-static positions from standing to squatting. Intersegmental hip forces, from a previous study, were applied to the subject-specific hip geometries, segmented from CT data, to evaluate the maximum shear stresses on the acetabular cartilage and underlying bone.

Results:

Peak maximum shear stresses were found at the anterosuperior region of the underlying bone during squatting. The peaks at the anterosuperior acetabulum were substantially higher for the patients (15.2 ± 1.8 MPa) in comparison with the controls (4.5 ± 0.1 MPa).

Conclusions:

Peaks were not situated on the cartilage, but instead located on the underlying bone. The results correspond with the locations of initial cartilage degradation observed during surgical treatment and from MRI.

Clinical Relevance:

These findings support the pathomechanism of cam FAI. Changes may originate from the underlying subchondral bone properties rather than direct shear stresses to the articular cartilage.     

The Shape of the Proximal Femur Influences Acetabular Wear Patterns Over Time

Background

Femoroacetabular impingement has been proposed as a cause of early osteoarthritis, but it is not known how this develops over time or whether the shape of the proximal femur influences this risk.

Questions/purposes

(1) Which areas of the acetabulum are worn more frequently by individuals with a cam deformity of the proximal femur? (2) Do observed acetabular wear patterns differ based on the etiology of the cam deformity? (3) Do wear patterns of individuals with a cam deformity differ based on an individual’s age?

Methods

We examined 645 corresponding femora and acetabuli from the Hamann-Todd Osteological Collection and determined the offset and alpha angle using photographs; 370 specimens met inclusion criteria and were examined for signs of wear and the locations of wear were recorded. Specimens were separated into eight subgroups based on age either younger than 40 years or older than 60 years, alpha angle greater or less than 55°, and degree of anterior head-neck offset. We compared the prevalence of wear between groups in each location.

Results

Individuals with abnormal geometry of the proximal femur demonstrated different wear patterns from individuals with normal geometry. There were few differences in wear patterns identified based on the etiology of the femoral deformity. Abnormal femoral geometry was associated with more frequent wear primarily at the anterosuperior acetabulum for individuals younger than 40 years of age and globally for individuals older than 60 years of age.

Conclusion

Femoral geometry appears to influence the pattern of acetabular wear occurring over time.     

Is the Acetabulum Retroverted in Slipped Capital Femoral Epiphysis?

Background

Recent biplanar radiographic studies have demonstrated acetabular retroversion and increased superolateral femoral head coverage in hips with slipped capital femoral epiphysis (SCFE), seemingly divergent from earlier CT-based studies suggesting normal acetabular version.

Question/purposes

We therefore asked: Are there differences in (1) acetabular version at the superior ¼ of the acetabular dome (AV_sup), (2) acetabular version at the center of the femoral head (AV_cen), and (3) superolateral femoral head coverage (lateral center-edge angle [LCEA]) among affected SCFE hips, unaffected hips, and normal controls?

Methods

We identified 32 patients with SCFE who underwent CT between 2007 and 2012. Twenty-three met our inclusion criteria. Seventy-six age- and sex-matched normal patients comprised the control group. Pelvic rotation, tilt, and inclination were corrected on each CT. AV_sup, AV_cen, and LCEA were measured.

Results

The mean AV_sup of the affected hips (−1.71°) demonstrated retroversion compared to the unaffected hips and the control group; the mean AV_sup of the unaffected hips was similar to that of the normal controls. Mean AV_cen was similar among the three groups. The LCEA was higher in affected and unaffected SCFE hips than in the control group (34.3° versus 34.5° versus 28.9°, respectively), but we found no difference between affected and unaffected hips.

Conclusions

Our data suggest an association of superior acetabular retroversion and increased superolateral femoral head coverage in SCFE. Whether this represents a primary abnormal morphology or a secondary pathologic response remains unclear. Further studies investigating the role of acetabular morphology in SCFE and its implications for development of symptomatic femoroacetabular impingement are warranted.     

Cross-sectional Anatomy of the Ilium: Implications for Acetabular Component Placement in Total Hip Arthroplasty

Background

High hip center reconstructions, used in revision and complex primary THAs, rely on pelvic bone stock at least 35 mm above the anatomic teardrop. However, the technique does not restore normal hip biomechanics and controversy exists regarding acetabular implant survival. Previous reports document a wide range of implant positioning above the teardrop. There is no anatomic guidance in the literature regarding the amount of bone stock available for initial implant stability in this area of the ilium.

Questions/purposes

We therefore determined the thickness of the human ilium and related it to acetabulum cup coverage in high hip center reconstructions.

Methods

We sectioned 16 cadaveric hips from the anterior superior iliac spine to the anatomic teardrop in 5-mm increments, then measured the thickness of the ilium for each cross section.

Results

The maximum thickness of 42 ± 9 mm occurred at the dome of the acetabulum 35 ± 3 mm above the teardrop. At a distance of 1 cm above the dome, the ilium was reduced by 24%, to 32 ± 6 mm. At 2 cm above the dome, the ilium thickness was 22 ± 4 mm, a 48% reduction from its maximum.

Conclusion

There are substantial anatomic limitations to high hip reconstructions 2 cm above the acetabular dome.     

Does Radiographic Coxa Profunda Indicate Increased Acetabular Coverage or Depth in Hip Dysplasia?

Background

Although radiographic coxa profunda has been considered an indicator of acetabular overcoverage, recent studies suggest that radiographic coxa profunda is a nonspecific finding seen even in hip dysplasia. The morphologic features of coxa profunda in hip dysplasia and the frequency with which the two overlap are not well defined.

Questions/purposes

We determined (1) the prevalence of radiographic coxa profunda in patients with hip dysplasia; (2) the morphologic differences of the acetabulum and pelvis between patients with hip dysplasia and control subjects; and (3) the morphologic differences between hip dysplasia with and without coxa profunda.

Methods

We retrospectively reviewed the pelvic radiographs and CT scans of 70 patients (70 hips) with hip dysplasia. Forty normal hips were used as controls. Normal hips were defined as those with a lateral center-edge angle between 25° and 40°. Coxa profunda was defined as present when the acetabular fossa was observed to touch or was medial to the ilioischial line on an AP pelvic radiograph. CT measurements included acetabular version, acetabular coverage, acetabular depth, and rotational alignment of the innominate bone.

Results

The prevalence of coxa profunda was 44% (31 of 70 hips) in dysplastic hips and 73% (29 of 40 hips) in the control hips (odds ratio, 3.32; 95% CI, 1.43–7.68). Dysplastic hips had a more anteverted and globally shallow acetabulum with inwardly rotated innominate bone compared with the control hips (p < 0.001). Dysplastic hips with coxa profunda had a more anteverted acetabulum (p < 0.001) and inwardly rotated innominate bone (p < 0.002) compared with those without coxa profunda, whereas the acetabular coverage and depth did not differ between the two groups, with the numbers available.

Conclusions

Radiographic coxa profunda was not a sign of increased acetabular coverage and depth in patients with hip dysplasia, but rather indicates classic acetabular dysplasia, defined by an anteverted acetabulum with anterolateral acetabular deficiency and an inwardly rotated pelvis. Thus, the presence of coxa profunda does not indicate a disease in addition to hip dysplasia, and the conventional maneuvers during periacetabular osteotomy are adequate for these patients.

Level of Evidence

Level IV, diagnostic study.     

Posterior acetabular arc angle of unstable posterior hip fracture–dislocation

Purpose

Posterior hip fracture–dislocation needs stability evaluation. A previous study in the normal acetabulum has shown that the coronal posterior acetabular arc angle (PAAA) could be used to assess an unstable posterior hip fracture. Our study was designed to assess PAAA of unstable posterior hip fracture–dislocation and whether posterior acetabular wall fracture involves the superior acetabular dome.

Methods

Using coronal computed tomography (CT) of the acetabulum and 3D reconstruction of the lateral pelvis, we measured coronal, vertical PAAA and posterior acetabular wall depth of 21 unstable posterior hip fracture–dislocations and of 50 % normal contralateral acetabula. Posterior acetabular wall fracture was assessed to determine whether the fracture involved the superior acetabular dome and then defined as a high or low wall fracture using vertical PAAA in reference to the centroacetabulo–greater sciatic notch line.

Results

The coronal PAAA of unstable posterior hip fracture–dislocations and of 50 % of the posterior acetabular wall of normal the contralateral acetabulum were 54.48° (9.09°) and 57.43° (5.88°) and corresponded to 15.06 (4.39) and 15.61 (2.01) mm of the posterior acetabular wall without significant difference (p > 0.05). The vertical PAAA of unstable posterior hip fracture–dislocation was 101.67° (20.44°). There were 16 high posterior acetabular wall fractures with 35.00 (16.18) vertical PAAA involving the acetabular dome and 5 low wall fractures. High posterior wall fractures resulted in four avascular necroses of the femoral head, three sciatic nerve injuries and one osteoarthritic hip.

Conclusion

Coronal and vertical PAAA of unstable posterior hip fracture–dislocations were 54.48° and 101.67°. Vertical PAAA assesses high or low posterior acetabular wall fracture by referring to the centroacetabulo–greater sciatic notch line. High posterior wall fracture seems to be the most frequent and is involved with many complications.     

Posterior acetabular arc angle of the femoral head assesses instability of posterior fracture-dislocation of the hip

Purpose

Unstable posterior fracture-dislocation of the hip is determined by the wall defect or acetabular fracture index. The unstable hip is a result of inadequate posterior acetabular coverage of the femoral head from the posterior acetabular wall fracture. In order to measure total posterior acetabular coverage of the femoral head and avoid using the contralateral acetabulum as a calculation reference, the posterior acetabular arc angle of the femoral head was measured to assess stability of posterior fracture-dislocation of the hip.

Methods

Using coronal computed tomography (CT) scan of the normal contralateral acetabulum at the level of the widest acetabular diameter and thinnest medial wall of 60 acetabular fractures, posterior acetabular arc angles of the femoral head in intact, 20 % and 50 % defects of posterior acetabular walls were measured. The angles were measured from the acetabular centre to the thinnest medial wall and to the top, inner cortex of 80 % and 50 % posterior acetabular walls.

Results

Average intact, 80 % and 50 % posterior acetabular walls were 33.82 ± 4.30, 26.88 ± 3.33 and 16.91 ± 2.15 mm which corresponded to 92.25 ± 11.34, 77.42 ± 10.04 and 50.63 ± 6.58° of posterior acetabular arc angles of the femoral head. The intraclass correlation coefficient (ICC) of the measurements including correlation of conversion of posterior acetabular wall depths to posterior acetabular arc angles of the femoral head were more than 0.82 and 0.89.

Conclusions

The measurement technique of posterior acetabular arc angle of the femoral head has strong reliability. Therefore, stable or unstable posterior fracture-dislocation of the hip can be determined in terms of more than 77 degrees or less than 50 degrees of posterior acetabular arc angles of the femoral head instead of less than 20 % or more than 50 % posterior acetabular wall defect.     

Effect of Pelvic Osteotomy in the Skeletally Immature on Acetabular Coverage

Background

Although pelvic osteotomy in children has been effective in re-establishing containment of the hip joint, its impact on hip joint development with respect to acetabular coverage is ill defined.

Purpose

The purpose of this study is to determine the prevalence of acetabular overcoverage in patients who had pelvic osteotomy during childhood and its impact on patient function.

Patients and Methods

Between 1980 and 2008, all patients who had a pelvic osteotomy done at our institution for non-neuropathic hip dysplasia (DDH) or secondary to Legg–Calvé–Perthes disease (LCP) prior to skeletal maturity were reviewed. A clinical assessment and the WOMAC, UCLA Activity Score, Marx activity score, and SF-36 quality-of-life questionnaires were completed. A standardized AP pelvic X-ray was performed to determine the acetabular coverage, signs of retroversion, and degenerative changes.

Results

Twenty-eight patients (32 hips) were identified, of which 14 (9 DDH, 5 LCP) agreed to participate. Impingement sign was positive in eight patients (six DDH, two LCP). Crossover and ischial spine signs were each present in ten hips. Tonnis grades were: 0 in 1 hip, 1 in 10 hips, 2 in 2 hips, and 3 in 1 hip. The mean Tonnis angle was 11.6 ± 8.6°. The mean CE angle was 24.0 ± 15.9° with six hips having a CE angle <20° and one hip with a CE angle >40°. There was no correlation between crossover sign or ischial sign and Tonnis grade (p = 0.739), hip pain (p = 0.520), or impingement sign (p = 1.00).

Conclusions

Acetabular overcoverage is common in patients who underwent pelvic osteotomy during childhood. No correlation was identified between retroversion and hip pain in our patient cohort.     

Effect of design on the initial stability of press-fit cups in the presence of acetabular rim defects: experimental evaluation of the effect of adding circumferential fins

Purpose

This experimental study was undertaken to examine the fixation characteristics of a six-finned acetabular cup in both primary and revision arthroplasty in comparison with two other commonly used cup designs without fins.

Methods

All three cup designs (Ananova® [Intraplant], Plasmacup® NSC [Aesculap]; Exceed ABT™ [Biomet]) were implanted into validated models of normal and revision acetabula. The defect models were designed to simulate a dorso-cranial rim defect of 90° width and 10 mm in depth (moderate rim defect) and a dorso-cranial rim defect of 130° width and 15 mm in depth (severe rim defect). The fixation strength of the three cup designs was tested by cyclically edge-loading the implanted cups using a mechanical testing machine.

Results

The six-finned Ananova implant exhibited greater resistance to foam-cup interface motion than both the Plasmacup and Exceed ABT implants. The largest average differences were observed in the resistance to ultimate spin-out, with Ananova outperforming Exceed ABT and Plasmacup by 26 % and 17 % in the moderate and by 36 % and 38 % in the severe defect models, respectively.

Conclusions

The six-finned Ananova cup was significantly more resistant to edge loading both in the normal acetabulum and in acetabula with moderate to severe dorso-cranial rim defects than cup designs without fins, indicating that it may cover a wider range of clinical indications than conventional press-fit cups and provide clinicians with the confidence that, in primary and simple revision arthroplasty, adequate fixation strength can be obtained.     

Reliability of Overcoverage Parameters With Varying Morphologic Pincer Features: Comparison of EOS® and Radiography

Background

Multiple radiographic parameters used for diagnosis and quantification of morphologic pincer features have emerged, but the degree to which pelvic tilt or rotation affects conventional radiography and EOS^® is unknown.

Question/purposes

We asked: (1) What is the reliability of EOS^® and conventional radiography at increasing sizes of morphologic pincer features with varying degrees of tilt and rotation? (2) What is the effect of tilt and rotation on acetabular overcoverage measurements?

Methods

Using a dry cadaveric pelvis, AP conventional radiographs and EOS^® images were taken at intervals of increasing modeled pincer size with 0° to 15° varying tilt and rotation. Lateral center-edge angle, Sharp angle, Tönnis angle, crossover sign, and retroversion index were measured on all images. Statistical analysis was conducted.

Results

The intermodality intraclass correlation coefficients for conventional radiography and EOS^® radiography across all pincer sizes, rotations, and tilts were excellent (0.93–0.98). Crossover sign was in perfect agreement in conventional radiography and EOS^®. Rotation of the hip away from the beam source and/or increased anterior tilt falsely increased all overcoverage parameters except for Tönnis angle. Rotation away from the beam of 10°or greater or anterior tilt of 5° or greater produced a false-positive crossover sign.

Conclusions

EOS^® radiography maintained excellent reliability in comparison to conventional radiography but both were equally vulnerable to the effects of tilt and rotation for quantification of hip parameters used in acetabular overcoverage assessment. A standardized pelvic radiograph ensuring that the pelvis is not excessively tilted or rotated should be used for assessing acetabular overcoverage parameters.     

Femoroacetabular Impingement: Prevalent and Often Asymptomatic in Older Men: The Osteoporotic Fractures in Men Study

Background

The epidemiology of femoroacetabular impingement (FAI) is important but incompletely understood, because most reports arise from symptomatic populations. Investigating the prevalence of FAI in a community-based cohort could help us better understand its epidemiology and in particular the degree to which it might or might not be associated with hip pain.

Questions/purposes

The purposes of this study were (1) to evaluate the proportion of older (≥ 65 years of age) men with morphologic abnormalities consistent with FAI; and (2) to assess the association of the morphologic abnormalities with prevalent radiographic hip osteoarthritis (OA) and hip pain.

Methods

Anteroposterior radiographs were obtained in 4140 subjects (mean age ± SD, 77 ± 5 years) from the Osteoporotic Fractures in Men study. We assessed each hip for cam, pincer, and mixed FAI types using validated radiographic definitions. Both intra- and interobserver reproducibility were > 0.9. Radiographic hip OA was assessed by an expert reader (intraobserver reproducibility, 0.7–0.8) using validated methods, and summary grades of 2 or greater (on a scale from 0 to 4) were used to define radiographic hip OA. Covariates including hip pain in the last 30 days were collected by questionnaires that were answered by all patients included in this report. Logistic regressions with generalized estimating equations were performed to evaluate the association of radiographic features of FAI and arthrosis.

Results

Pincer, cam, or mixed types of radiographic FAI had a prevalence of 57% (1748 of 3053), 29% (886 of 3053), and 14% (419 of 3053), respectively, in this group of older men. Both pincer and mixed types of FAI were associated with arthrosis but not with hip pain (odds ratio [OR], 1.63; 95% confidence interval [CI], 1.25–2.13; p < 0.001 for pincer and OR, 2.49; 95% CI, 1.65–3.76; p < 0.001 for mixed type). Patients with hips characterized by cam-type FAI had slightly reduced hip pain without the presence of arthrosis compared with hips without FAI (OR, 0.82; 95% CI, 0.68–0.99; p = 0.037). A center-edge angle > 39° and a caput-collum-diaphyseal angle < 125° were associated with arthrosis (OR, 1.53; 95% CI, 1.22–1.94; p < 0.001 and OR, 2.09; 95% CI, 1.24–3.51; p = 0.006, respectively), but not with hip pain (OR, 0.89; 95% CI, 0.77–1.03; p < 0.108 and OR, 0.99; 95% CI, 0.67–1.45; p = 0.945, respectively). An impingement angle < 70° was associated with less hip pain compared with hips with an impingement angle ≥ 70° (OR, 0.76; 95% CI, 0.61–0.95; p = 0.015).

Conclusions

FAI is common in older men and represents more of an anatomic variant rather than a symptomatic disease. This finding should raise questions on how age, activities, and this anatomic variant each contribute to result in symptomatic disease.

Level of Evidence

Level III, prognostic study.     

Three dimensional-CT evaluation of femoral neck anteversion, acetabular anteversion and combined anteversion in unilateral DDH in an early walking age group

Purpose

At present, the indications for femoral derotational osteotomy remain controversial due to the inconsistent findings in femoral neck anteversion in developmental dysplasia of the hip (DDH). Moreover, combined anteversion is not assessed in unilateral DDH using three dimensional-CT. Therefore, the purposes of our study were to observe whether the femoral neck anteversion (FA), acetabular anteversion (AA) and combined anteversion (CA) on the dislocated hips were universally presented in unilateral DDH according to the classification system of Tönnis.

Methods

Sixty-two patients with unilateral dislocation of hip were involved in the study, including 54 females and eight males with a mean age of 21.63 months (range, 18–48 months). The FA, AA and CA were measured and compared between the dislocated hips and the unaffected hips.

Results

Although no significant difference was observed in FA between the dislocated hips and the unaffected hips (P = 0.067, 0.132, respectively) in Tönnis II and III type, FA was obviously increased on the dislocated hips compared with the unaffected hips in Tönnis IV type. Increased AA on the dislocated hips was a universal finding in Tönnis II, III and IV types. Meanwhile, a wide safe range of CA from 24° to 62° was demonstrated on the unaffected hips.

Conclusion

Femoral derotational osteotomy seems not to be necessary in Tönnis II and III types in unilateral DDH. Femoral derotational osteotomy should be considered in DDH, especially in Tönnis IV type, if the CA is still above 62° and the hip joints present instability in operation after abnormal acetabular anteversion, acetabular index and acetabular coverage of the femoral head are recovered to normal range through pelvic osteotomy.     

Surgical hip dislocation for treatment of cam femoroacetabular impingement

Background:

Cam femoroacetabular impingement is caused by a misshapen femoral head with a reduced head neck offset, commonly in the anterolateral quadrant. Friction in flexion, adduction and internal rotation causes limitation of the hip movements and pain progressively leading to labral and chondral damage and osteoarthritis. Surgical hip dislocation described by Ganz permits full exposure of the hip without damaging its blood supply. An osteochondroplasty removes the bump at the femoral head neck junction to recreate the offset for impingement free movement.

Materials and Methods:

Sixteen patients underwent surgery with surgical hip dislocation for the treatment of cam femoroacetabular impingement by open osteochondroplasty over last 6 years. Eight patients suffered from sequelae of avascular necrosis (AVN). Three had a painful dysplastic hip. Two had sequelae of Perthes disease. Three had combined cam and pincer impingement caused by retroversion of acetabulum. All patients were operated by the trochanteric flip osteotomy with attachments of gluteus medius and vastus lateralis, dissection was between the piriformis and gluteus minimus preserving the external rotators. Z-shaped capsular incision and dislocation of the hip was done in external rotation. Three cases also had subtrochanteric osteotomy. Two cases of AVN also had an intraarticular femoral head reshaping osteotomy.

Results:

Goals of treatment were achieved in all patients. No AVN was detected after a 6 month followup. There were no trochanteric nonunions. Hip range of motion improved in all and Harris hip score improved significantly in 15 of 16 cases. Mean alpha angle reduced from 86.13° (range 66°–108°) to 46.35° (range 39°–58°).

Conclusion:

Cam femoroacetabular Impingement causing pain and limitation of hip movements was treated by open osteochondroplasty after surgical hip dislocation. This reduced pain, improved hip motion and gave good to excellent results in the short term.