Fig. 4.1
Measured range of motion in normal children and children with DDH. [Reprinted from Rao KN, Joseph B. Value of Measurement of Hip Movements in Childhood Hip Disorders. J Pediatr Orthop [Internet]. 2001;21(4):495–501. With permission from Wolters Kluwer Health.]
Stress
Generally, the results from mathematical models show that higher hip contact stress is associated with more severe deformity and less positive outcomes. Increased deformity was associated with increased peak contact stress (Severin 1a and 1b: 2.3 MPa; Severin 2a and 2b: 2.4 MPa; Severin 3: 4.6 MPa) in a study using models whose geometry came from AP radiographs for 35 patients [11]. Peak stresses in both the femur and acetabulum increased with increasing deformity [12] in a study using a three-dimensional finite element analysis (FEA) of a CT-based normal hip joint model which was deformed to simulate three severities of DDH. Peak contact stress increased in hips with less acetabular coverage when the abductor muscle force became more vertical in models of dysplastic hips based on modifications to normal 2D models created from AP radiographs [13].
A study that used planar models from X-rays combined with duration of follow-up (mean 29 years) to calculate accumulated stress over time in 89 DDH hips identified a damage threshold of 10 MPa-years, finding that 80.9 % of all hips below the threshold had good outcomes based on Severin classifications, and 90.4 % of all hips above the threshold had poor outcomes [14]. Two non-uniform contact stress models based on longitudinal radiographic information from the same 89 DDH hips showed an association between higher loads and worse clinical outcomes, although the damage thresholds (based on clinical outcomes) were very different for the two models (2.0 MPa versus 4.5 MPa) [15].
Mathematical models have shown that various osteotomies used to treat DDH reduce peak stress or contact stress. Models based on radiographs showed that the Tonnis osteotomy for insufficient coverage and avascular necrosis of the femoral head reduced peak normalized contact stress by 55.9 % (peak stress/BW) in 75 patients [16]. A 2D model showed that triple osteotomy of the innominate bone decreased contact stress and increased contact area, although not to the level of normal controls [17]. A 3D model showed that the Bernese periacetabular osteotomy for residual dysplasia increased the normalized resultant hip force but reduced the peak contact stress normalized by BW from 5.2 to 3.0 kPa/N due to increased coverage [18]. Better long-term clinical outcomes were observed in hips with lower postoperative normalized peak stress [18]. The same 3D FEA model mentioned above was used to simulate a Bernese periacetabular osteotomy in each severity level of DDH. Peak stresses were found to decrease with osteotomy in both the femur and acetabulum, although none were reduced to the level of the normal model, and more severe deformity was associated with higher stress following osteotomy [12].
Force
Salter osteotomy reduced the measured resultant joint force on the hip from 2.7 BW (583 N) to 1.2 BW (266 N) in a plastic model of a patient’s DDH joint created with rapid prototyping [19]. Mean gluteus maximus force was similarly reduced from 0.46 BW (100 N) to 0.24 BW (52 N).
General/Focal Acetabular Overcoverage
General acetabular overcoverage is characterized by a very deep acetabulum or circumferentially prominent acetabular rim [20]. Coxa profunda and protrusio acetabuli are defined by overlapping of the ilioischial line medial to the acetabular fossa or femoral head, respectively, on an AP radiograph. A center-edge (CE) angle below 25° is associated with dysplasia, while a CE angle above 39° describes overcoverage [20, 21].
The main biomechanical failure mechanism in general overcoverage (coxa profunda/protrusio acetabuli) is hypothesized to be dynamic pincer-type femoroacetabular impingement, which is associated with osteoarthritis. General acetabular overcoverage is often referred to as a pincer deformity. The mechanism of pincer impingement is thought to be characterized by linear contact of the femoral head-neck junction against the acetabular rim and labrum [22, 23]. Chondrolabral damage patterns related to pincer morphology are widely distributed around the acetabulum [24].
Focal acetabular overcoverage is characterized by a prominence of the acetabular rim in a specific location, and is often related to acetabular retroversion. In retroversion, the acetabular opening is oriented more posteriorly than normal [25]. Clinically, retroverted acetabula are commonly associated with posterior cartilage damage and anterior impingement-related chondrolabral pathology [25–27]. Retroversion is often indicated by the cross-over sign on a plain A-P radiograph, or can be determined using 3D CT [28].
As with general overcoverage, the main biomechanical mechanism of concern in retroverted hips is thought to be pincer impingement at the anterior rim. Therefore, retroversion is frequently combined with protrusio/profunda in the pincer impingement literature. It is important to note that some retroverted acetabula are associated with deficient posterior coverage [28, 29], and subsequently may have different static loading patterns compared to profunda/protrusio acetabula.
Range of Motion
In an in vivo study, 32 hips with cam or pincer pathoanatomy had a mean internal rotation ROM at 90° flexion of 4° ± 8° (range: −10° to 20°) compared to 28° ± 7° (range: 10–40°) in 40 control hips [30]. This study also quantified the neck-rim relationship on open-configuration MRI scans taken with hips in 90° of flexion using the β angle, defined by a line connecting the femoral head center to the head-neck junction, and a line connecting the femoral head center to the acetabular margin (Fig. 4.2). The mean β angle was only 5° ± 9° in the cam or pincer subjects compared to 30° ± 9° in the controls. This work supports the hypothesis that linear abutment of the head-neck junction against the acetabular rim (impingement) terminates motion.
Fig. 4.2
(Top) Definition of the β angle on an MR image (Bottom) and on a diagram. [Reprinted from Wyss TF, Clark JM, Weishaupt D, Nötzli HP. Correlation between internal rotation and bony anatomy in the hip. Clin Orthop Relat Res [Internet]. 2007 Jul [cited 2013 Feb 5];460(460):152–8. With permission from Wolters Kluwer Health.]
Computer models confirm that most types of cam and/or pincer pathomorphology lead to reductions in flexion, internal rotation, abduction, and internal rotation at high flexion, although pincer and cam deformity have often been assessed together. Internal rotation at 90° flexion, a representation of the anterior impingement test, is commonly simulated with these models because it is thought to bring the anterior femoral head-neck junction close to the anterosuperior quadrant of the acetabular rim (often the most prominent part of the rim and a common site for chondrolabral pathology).
In a study using a mathematical model, 31 symptomatic hips with cam or pincer pathoanatomy (12 cam, 7 pincer, 12 mixed) had significantly decreased flexion, internal rotation at 90° of flexion, and abduction compared to a control group of 36 hips [31]. The same model was used to predict the location of impingement during internal rotation at high flexion in six hips with pincer deformities. The predicted impingement site for the pincer group was highly localized anterosuperiorly, whereas the actual site of chondral and labral damage observed in a separate group of 16 pincer hips spanned nearly the entire superior portion of the acetabulum and extended inferiorly [24].
Another model predicted that a group of 10 pure cam hips, 8 pure pincer hips, and 10 with combined cam/pincer pathoanatomy had limited flexion, internal rotation, abduction and internal rotation at 90° of flexion compared to 33 normal hips. The model predicted impingement on the anterosuperior quadrant of the acetabular rim for both control and FAI hips, with minimal difference in impingement zones between cam/pincer/combined hips [32].
Models of 50 hips undergoing arthroscopy for FAI showed that increased acetabular retroversion (simulated with increased anterior tilt) decreased internal rotation ROM by 5.9° at 90° flexion, and by 8.5° at 90° flexion plus 15° adduction. Increased retroversion shifted the predicted impingement zone anteriorly. Increased acetabular anteversion (simulated with increased posterior tilt) increased internal rotation ROM by 5.1° at 90° flexion, and by 7.4° in FADIR [33].
Stress
Using idealized hip geometry in a finite element model, Chegini et al. evaluated the effects of varying α angles (40–80° range at 10° intervals) and center-edge (CE) angles (0–40° range at 10° intervals) on hip joint contact stress and acetabular cartilage stress during stand-to-sit and walking from heel-strike to toe-off [34]. Overcoverage reduced contact stress and chondral stress, while dysplasia greatly increased contact stress and chondral stress. Peak joint contact stress as well as chondral stress was inversely related to CE angle. Conversely, at deep flexion during stand-to-sit, a high α angle in combination with a high CE angle yielded the highest contact stress and acetabular chondral stress (Fig. 4.3). Cartilage stress distribution in the mixed cam-pincer hip during stand-to-sit was concentrated in the anterosuperior quadrant, where intraoperative cartilage damage is often observed (Fig. 4.4). These results indicate that generally overcovered hips are likely not at risk for highly stressed posterior acetabular cartilage.
Fig. 4.3
Effect of CE angle and alpha angle on maximum von Mises stress in acetabular cartilage during stand-to-sit. [Reprinted from Chegini S, Beck M, Ferguson SJ. The effects of impingement and dysplasia on stress distributions in the hip joint during sitting and walking: a finite element analysis. J Orthop Res [Internet]. 2009 Feb [cited 2014 Jun 18];27(2):195–201. With permission from John Wiley & Sons, Inc.]
Fig. 4.4
(Top) Intraoperative image of cartilage damage in a cam-type hip. (Bottom) Cartilage stress distribution predicted by a model of a typical cam-type hip during stand-to-sit. [Reprinted from Chegini S, Beck M, Ferguson SJ. The effects of impingement and dysplasia on stress distributions in the hip joint during sitting and walking: a finite element analysis. J Orthop Res [Internet]. 2009 Feb [cited 2014 Jun 18];27(2):195–201. With permission from John Wiley & Sons, Inc.]
The applied forces and resulting peak stresses (3.3–16.5 MPa) from this model are within the same range reported by studies using CT-based patient-specific geometry for normal hips [35–38]. The contact stress magnitudes are consistent with experiments where miniature pressure transducers implanted superficially into normal cadaver femoral head cartilage measured average peak contact stress in femoral cartilage to be 8.8 MPa for an applied vertical force load of 2700 N [39].
Subject-specific finite element models showed that contact stress was concentrated in the superomedial (SM) region in retroverted acetabula, while normal hips had more widely distributed contact stresses (Fig. 4.5). During walking and stair descent, normal hips had 2.6–7.6 times larger contact stresses in the posterolateral (PL) acetabulum. Conversely, retroverted hips had 1.2–1.6 times larger contact stresses in the superomedial acetabulum [37]. The authors suggest that these results refute the theory of high posterior stresses in retroverted acetabula due to decreased posterior coverage. A lack of concentrated loads on the posterior acetabulum suggests that retroverted hips with cartilage degradation on the posterior acetabulum may more likely be due to levering and “contre-coup” contact, rather than static posterior overload.
Fig. 4.5
Acetabular contact stress predictions for normal and retroverted hips for walking mid-stride (WM), descending stairs (DH) and chair rising (CR). The arrows indicate the approximate direction and relative magnitude of the load during each activity. [Reprinted from Henak CR, Carruth ED, Anderson a E, Harris MD, Ellis BJ, Peters CL, et al. Finite element predictions of cartilage contact mechanics in hips with retroverted acetabula. Osteoarthritis Cartilage [Internet]. Elsevier Ltd; 2013 Oct [cited 2014 Jun 17];21(10):1522–9. With permission from Elsevier.]
Posterior Overcoverage: Acetabular Anteversion
Acetabular anteversion is characterized by an acetabular opening that projects anteriorly. A prominent posterior wall may be associated with acetabular anteversion and might reduce the available bony range of extension and external rotation. Further, the anterior wall might be deficient and lead to overload. However, we found no biomechanics studies that evaluated the effects of posterior overcoverage.
Biomechanical Effect of Problems in Femoral Neck Orientation
Femoral Anteversion and Coxa Valga
A valgus femur, or femur with coxa valga, is characterized by a caput-collum-diaphyseal (CCD) angle greater than 135° [40, 41]. Coxa valga is hypothesized to arise secondary to DDH and as such is associated with concentrated stresses on the acetabular roof. Femoral anteversion (or antetorsion) is characterized by a posteriorly oriented femoral neck, which is closer than normal to the posterior acetabulum and related acetabular structures. Recent work has focused on dynamic posterior impingement-related considerations in coxa valga and femoral anteversion.
Range of Motion
In an experimental study in children, hips with femoral anteversion had reduced external rotation in extension and abduction compared to normal subjects (Fig. 4.6) [10].
Fig. 4.6
Measured range of motion in hips with femoral anteversion (FA) and normal hips. [Reprinted from Rao KN, Joseph B. Value of Measurement of Hip Movements in Childhood Hip Disorders. J Pediatr Orthop [Internet]. 2001;21(4):495–501. With permission from Wolters Kluwer Health.]
Mathematical models predicted reduced adduction, extension and external rotation range of motion in hips with both coxa valga and anteversion [42]. External rotation at 90° of flexion was also limited. These findings are consistent with experimental measurements of hip range of motion [10, 40]. The authors suggested that femurs with both coxa valga and anteversion are predisposed to a reduced range of external rotation and extension due to posterior extra-articular impingement. The findings suggest that hips with coxa valga and high femoral anteversion are at substantial risk for posterior impingement, and that treatment decisions involving coxa valga/anteversion should consider dynamic pathology in addition to static overload. Extra-articular structures like the anterior inferior iliac spine, ischial tuberosity, greater trochanter, and lesser trochanter caused terminal impingement much more frequently in the coxa valga/anteversion group than in the control group. The authors postulate that posterior impingement may induce a levering effect and eventually cause “contre-coup” chondrolabral lesions on the anterosuperior acetabulum—which would explain positive anterior impingement tests.
Femoral Retroversion and Coxa Vara
Coxa vara is characterized by a caput-collum-diaphyseal (neck-shaft) angle less than 125° [43, 44]. In coxa vara, the superior margin of the femoral neck is closer than normal to the anterosuperior acetabulum and therefore associated with loss of hip ROM. Similarly, femoral retroversion (or retrotorsion) brings the anterior margin of the femoral neck closer to the anterosuperior acetabulum. Femoral version is defined in the axial or transverse plane by the angle between the femoral neck axis proximally and intercondylar line distally.
Range of Motion
Experimental measurements showed reduced abduction, internal rotation, and external rotation ROM in hips with infantile coxa vara compared to normals (Fig. 4.7) [10]. Hips with femoral retroversion had reduced internal rotation ROM and increased external rotation ROM compared to normals (Fig. 4.8) [10]. Hips with combined retroversion and coxa vara had substantially reduced abduction and internal rotation ROM and slightly increased external rotation than normal (Fig. 4.9) [10].
Fig. 4.7
Range of motion in normal hips and hips with infantile coxa vara (ICV). [Reprinted from Rao KN, Joseph B. Value of Measurement of Hip Movements in Childhood Hip Disorders. J Pediatr Orthop [Internet]. 2001;21(4):495–501. With permission from Wolters Kluwer Health.]
Fig. 4.8
Range of motion in normal hips and hips with femoral retroversion (FR). [Reprinted from Rao KN, Joseph B. Value of Measurement of Hip Movements in Childhood Hip Disorders. J Pediatr Orthop [Internet]. 2001;21(4):495–501. With permission from Wolters Kluwer Health.]
Fig. 4.9
Range of motion in normal hips and hips with coxa vara and femoral retroversion (R.V./C.V.). [Reprinted from Rao KN, Joseph B. Value of Measurement of Hip Movements in Childhood Hip Disorders. J Pediatr Orthop [Internet]. 2001;21(4):495–501. With permission from Wolters Kluwer Health.]
Although it is not clear that bony collisions terminate motion, this work demonstrates that abnormal morphology that brings the femoral neck closer to the acetabular rim in a specific plane is associated with limited ROM in that plane. In coxa vara, the femoral neck is brought closer to the superior acetabulum in the coronal plane, and likewise motion towards the superior acetabulum in the coronal plane—abduction—is limited. Similarly in retroversion, the femoral neck is brought closer to the anterior acetabulum, and likewise motion towards the anterior acetabulum in the axial plane—internal rotation—is limited. For this reason, it is hypothesized that coxa vara and femoral retroversion increase the likelihood of linear impact of the femoral neck against the acetabulum (i.e., pincer impingement) occurring during daily activity.
Biomechanical Effect of Problems at the Femoral Head-Neck Junction
SCFE
Slipped capital femoral epiphysis (SCFE) is a primarily adolescent disorder where the epiphysis slips in an inferior and posterior direction along the growth plate or physis, resulting in a femoral deformity believed to lead to acetabular impingement and cartilage damage [45].
Range of Motion
SCFE is expected to cause loss of ROM due to impingement of the deformed femoral head or neck on the acetabulum.
In a study using computer models from 31 SCFE patients and 15 contralateral controls [46], mild slips (as defined by Southwick angle) generally showed similar or slightly reduced ROM compared to controls (e.g., flexion: mild SCFE 89°, normal 99°), while severe slips had drastic reductions in ROM (e.g., flexion: severe SCFE 4°). For mild SCFE with a more prominent head-neck junction (type 2), ROM was further decreased (e.g., flexion: mild SCFE (type 2) 62°). Moderate SCFE cases were associated with larger decreases in ROM (e.g., flexion: moderate SCFE 14.2°), which were worsened by prominent head-neck junctions (e.g., flexion: moderate SCFE (type 3) 2°), while severe slips were not further affected by head-neck junction morphology.
Stress
Finite element models for both hips from two unilateral SCFE patients (one moderate, one severe) predicted that for a moderate slip, the peak contact stress was 17 % higher and maximum stress was 29 % higher than in the contralateral hip [45]. In the severe slip, peak contact stress was 49 % higher and maximum stress was 170 % higher. Simulated subcapital osteotomy through the proximal femoral epiphysis, base-of-neck osteotomy at the neck outside the capsule, and intertrochanteric osteotomy between the greater and lesser trochanter did not change contact stress or stress for the moderate slip, while the severe case saw reductions in maximum stress by about half (along with smaller reductions in contact stress), although this was still higher than the contralateral normal hips.
Cam Deformity
A cam deformity is typified by decreased concavity of the femoral head-neck junction. Cam deformities are widely thought to increase the risk of hip osteoarthritis. In 1965, Murray identified a “tilt deformity” of the femoral head and noted that radiographic tilt deformities were present in 79 out of 200 cases of hip OA [47]. In 1975, Stulberg et al. described the similar “pistol-grip deformity,” and in 1976 Solomon postulated that hip OA was secondary to such deformities [48]. In 2003, Ganz proposed that hips with tilt/pistol-grip deformities, resembling mechanical “cams,” mainly fail due to cam-type femoroacetabular impingement (Fig. 4.10) [22]. It was postulated that the cam deformity jams inside the acetabulum during forceful motion, particularly internal rotation at high flexion [22], leading to concentrated shear forces on intra-articular cartilage and acetabular labrum. The theory was largely driven by intraoperative findings from more than 600 surgical dislocations of the hip [22, 23], and evidence that patients with acetabular rim syndrome frequently have reduced concavity at the femoral head-neck junction [50, 51].
Fig. 4.10
Schematic diagrams of cam (top) and pincer (bottom) impingement. [Reprinted from Ganz R, Leunig M, Leunig-Ganz K, Harris WH. The etiology of osteoarthritis of the hip: an integrated mechanical concept. Clin Orthop Relat Res [Internet]. 2008 Feb [cited 2014 Jul 10];466(2):264–72. With permission from Springer Verlag.]
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