Clinically, it may be challenging to differentiate various etiologies of hip dysfunction such as developmental dysplasia of the hip (DDH), isolated labral tears (LTs), and femoroacetabular impingement (FAI).1,2 Several imaging modalities are available to physicians when evaluating patients with prearthritic hip pain, each providing various degrees of information on the osseous structure as well as intra- and extra-articular soft tissues. Conventional radiography (x-ray) is an excellent first-line test to evaluate gross morphology of joint, bone shape, and orientation. However, primary bony abnormalities may be less conspicuous if standard radiographic measures of dysmorphism are borderline or if the deformities are subtle. Although the advent of 3-dimensional computer tomography provides a more in-depth analysis of the bony structures, it is unclear if they are more accurate in providing a diagnosis than standard radiographic techniques. More importantly, the soft tissue structures such as the labrum, capsule, or muscles surrounding the hip joint can vary depending on the bony architecture, either primarily or as a secondary adaptation thus possibly providing further insight into the underlying pathology.3,4 Some authors have reported on the use of magnetic resonance imaging as a more comprehensive modality providing optimal characterization of intra- and extra-articular soft tissue structures.4–6 Current literature tends to recognize that dysplastic hips will present with an enlarged labrum compared with patients with FAI.7 A more recent study suggested the use of an iliocapsularis-to-rectus-femoris ratio to differentiate between FAI and DDH, further emphasizing the role thus providing increasing evidence that soft tissue structure could provide additional findings that are pathognomonic of hip dysplasia.8 The purpose of this study was therefore to determine whether magnetic resonance arthrography (MRA) assessment of soft tissue structures of the hip (capsule, labrum, and muscle) differ in patients who have undergone surgical correction for dysplasia, CAM-FAI, and isolated LTs.
PATIENTS AND METHODS
Searching our prospectively created database of joint preserving surgery of the hip, 3 groups of patients who had a diagnosis of the 3 most common prearthritic hip deformities based on their hip surgery and clinical presentation were retrospectively identified: DDH, isolated LTs, and cam-type FAI. Although most FAI have mixed features of cam and pincer type, we purposefully selected only cam-type FAI hips to avoid comparing pincer-type FAI with LT hips that could have also presented with pincer FAI, therefore making it hard to find differences between the 2 groups. Inclusion criteria were as follows: for the DDH group, lateral center-edge angle (LCEA) less than 20 degrees preperiacetabular corrective osteotomy ; for the isolated LT group, an LCEA of more than 25 degrees and less than 40 degrees, with an alpha angle of less than 50 degrees on all imaging (radial MRA and plain x-rays) and presence of LT on MRA; and for the cam-type FAI group, corrective CAM surgery with a preoperative alpha angle >55 degrees and LCEA between 25 and 39 degrees.9–14 The study was approved by the institutional research ethics board (protocol #20140518-01H). Fifty-seven (57) patients who underwent corrective hip surgery between 2006 and 2015 and had a preoperative MRA were identified: 17 with DDH (11 F, 6 M; mean age 35.1 years, range 19.6–53.6), 20 with isolated LT (17 F, 3 M; mean age 38.4 years, range 15.2–62.1), and 20 with cam-type FAI (11 F, 9 M; mean age 38.8 years, range 18.9–51.2).
The MRA protocol is as follows: 8–15 mL of 2 mMol gadolinium saline solution were injected into the hip joint under fluoroscopic guidance, followed by 1.5 tesla(T) magnetic resonance imaging (Magnetom Symphony; Siemens Medical Solutions, Erlangen, Germany), with a large, flexible, receive-only, surface coil wrapped around the hip. Subjects were positioned supine with the legs fixed in neutral rotation using a matrix of 448 × 224 and slice thickness set at 3.5 mm. The studies included oblique axial T1, oblique coronal and oblique sagittal T1 fat suppressed (FS), and axial proton density (PD) FS sequences. The images were sent to the local PACS (picture archiving and communication system) for review, with measurements made with digital calipers available on PACS. A clockface nomenclature was used with superior position being 12 o'clock (12:00) and the anterior position being 3 o'clock (3:00). The hip capsule thickness and labral width were measured superiorly (12 o'clock) on the oblique coronal sequence through the mid-acetabulum and anteriorly (3 o'clock) on the oblique axial sequence through the mid-femoral neck. The labrum was measured along its long axis that is the longest dimension/length (Fig. 1), whereas the capsule was measured along its short axis that is the shortest dimension/thickness (Fig. 2). Several muscular dimensions were measured at standardized levels on axial proton density-FS images: iliopsoas transverse (trans) and anteroposterior (AP) at the level of mid-acetabulum, rectus femoris AP and trans at the level of inferior rim of acetabulum, and gluteus muscles short axis thickness at the level of the acetabular roof (Fig. 3). The AP and trans measures refer to short and long axes of the muscle bellies, respectively (Fig. 4). To account for differences in body habitus between subjects, femoral neck widths were measured on respective oblique axial slices that provided the widest measurements. Each parameters recorded were then divided by respective femoral widths for normalization in absolute values.15,16 All MRA readings were completed by a fellowship-trained musculoskeletal radiologist and a medical student specifically trained for the purpose of this study. Quantitative data are expressed as mean ± SD calculated for all variables. Before statistical analysis, all measurements were normalized relative to the respective femoral neck width as described above. Between-group analysis with analysis of variance was performed using SPSS with the level of significance set at P < 0.05. Interreader reliability was evaluated using intraclass correlation coefficient (ICC) on a randomly chosen sample of 15 cases. Landis and Koch (1977) guidelines to ICC interpretation were used consequently setting 0.21 to 0.40 as fair agreement, 0.41 to 0.60 as moderate agreement, 0.61 to 0.80 as substantial agreement, and 0.81 to 1.0 as almost perfect or perfect agreement.17
Normalized descriptive data are presented as mean ± SD in Tables 1 and 2. The superior capsule thickness of the DDH group [normalized value (NV): 0.24] was significantly greater compared with the LT group (NV: 0.15, P < 0.05) and the FAI group (NV: 0.16, P < 0.05) (Table 1). In addition, the dysplasia group had a significantly greater anterior capsular thickness (NV: 0.18) compared with the LT group (NV: 0.13, P < 0.05). There were significant differences between the dysplasia group (NV: 1.39) and the FAI group (NV: 1.13, P < 0.05) for the rectus femoris transverse axis (Table 2). When examining the labrum, superior labral length was larger in the DDH group (NV: 0.31) compared with the FAI group (NV: 0.25, P < 0.05), and the anterior labral width was larger in the hip dysplasia group (NV: 0.28) compared with the FAI group (NV: 0.24) and the LT group (NV: 0.24), although not significant (Table 1). There was no significant difference between the LT and FAI groups. The ICC of all measured variables ranged from a fair to almost perfect agreement and is presented in Table 3.
Although the bony anatomy will certainly remain the key to understanding the pathoanatomy of early degenerative disease of the hip, little is known of the associated soft tissue and muscular abnormalities or adaptations associated with hip deformities such as FAI and dysplasia. More importantly, these nonosseous abnormalities can impact patient function as well as both nonsurgical and surgical interventions as well as recovery from surgery. They may also provide further insight into how to best optimize management from diagnosis to surgical intervention. This is evident as greater focus is being placed on functionality after joint preserving surgery of the hip and minimizing the risk of revision surgery. In our study, we found significant differences in muscle, capsular, and labral size measurements between the 3 patient groups.
More specifically, MRA revealed a larger superior labrum in the DDH group compared with the FAI group. Our results coincide with those of Leunig et al (2004) who noted an enlargement of the acetabular labrum in patients with DDH and the absence of labral hypertrophy in patients with FAI.18 In that sense, our results complement those of Leunig by reiterating the notion that a deficient acetabular coverage can lead to secondary hypertrophic changes in the labrum that are mainly due to induced chronic stresses in the hip joint. In addition, we found a significant thicker superior capsule in the DDH group compared with the FAI and LT groups. These findings are again consistent with Leunig impression of altered hip biomechanics that can affect the soft tissue envelope including capsular changes as noted in our study. In other words, the underdevelopment of the acetabulum in dysplasia maintains a state of hip instability thus provoking a compensatory overgrowth of labrum and capsule thickening early in development as a means of passive stabilization. Therefore, this compensatory mechanism reinforces the need to limit capsular release and ensure good capsular repair within patients of the DDH group undergoing periacetabular osteotomy. Of note, more recent literature also echoes the same conclusions, Kubo et al (2015) and Gupta et al (2015) by direct visualization through arthroscopy.19,20
Consequently, differences observed in transverse dimension of rectus femoris also suggest that varying morphology of hip joints may lead to altered gait biomechanics, therefore affecting the activation and use of individual muscles with selective hypertrophy. This finding is consistent with those of Babst et al (2011) who suggested that the iliocapsularis would be hypertrophied in dysplastic hips as the muscle seems to actively assist constriction of the femoral head in deficient acetabulum.21,22 Following this line of reasoning, it is possible that the rectus femoris also has a similar function. The close anatomical proximity and similar muscle origin with respect to the iliocapsularis bring additional weight to support findings of larger rectus femoris in the DDH group. It is also possible that the additional stability provided by adequate femoral head coverage by the acetabulum in the FAI group contributes to a relatively decreased transverse dimension of the rectus femoris in the FAI group implying that there is variation in the degree to which various muscles are activated or used depending on the hip morphology. In addition, Haefeli et al (2015) suggested the use of an iliocapsularis-to-rectus-femoris ratio for several parameters such as thickness and width using a radial line passing through the femoral head center.8 Our assessment of muscle dimensions differed from this study as we measured the short and long axes with respect to the muscle orientation therefore trying to better assess the various muscle volumes and variability between the groups. This difference in methodology outlines the complexity surrounding the use of proper techniques for muscle measurements because several factors can influence the resultant observed dimensions. Further prospective gait analysis studies are required to better describe how hip morphology can affect muscle activation and hypertrophy and whether these change after surgical correction. In addition, this could help refine the nonsurgical management of patients with hip pain and optimize rehabilitation.
An effort was made to select age-matched patients to form 3 homogeneous study groups based on the sole diagnosis of DDH, FAI, or LT. The DDH group thus has an age slightly below average than that of FAI and LT groups. No sex subanalysis was possible because of the limited sample size, but normalization was performed using femoral neck width. Another limitation is that the isolated labral treatment group could still have subtle osseous abnormalities that were unrecognized by our current imaging modalities. Interestingly enough, most differences were noted between the dysplasia versus the FAI and LT groups, whereas the LT versus the FAI group had no significant differences. The lack of differentiation in the soft tissue envelope between the FAI and LT groups could be due to our relative small numbers but most likely reflective of the pathomechanism of overuse in CAM-FAI patients. More importantly, the natural history of CAM is not fully understood where a large proportion of patients can remain asymptomatic.18,23 Possible traction forces at the labrochondral junction in CAM-FAI hips could also result in chondral flap tears and therefore present with morphological similarities to LT hips. This hypothesis is supported by McCarthy et al, who found that 73% of hips with LTs had adjacent acetabular chondral damage.24 In addition, Johnston et al confirmed that CAM-FAI with high alpha angles were associated with labral injury and chondral damage.25
The inter-reader agreement (ICC) varied considerably depending on which hip component was measured. ICC variability (fair to strong; ICC 0.4–0.9) can be explained by the way measurements were made. Additional use of muscle measurements such as circumference and cross-sectional area could have provided better characterization in muscle dimensions and alleviate the ICC variability. Needle degradation anteriorly caused by contrast injection was a systemic issue across all subjects. This might have affected the accuracy of measurements by making it harder to properly visualize capsule, thus adding potential error sources in capsular measurements. Finally, 3-dimensional dynamic imaging modalities are becoming increasingly used in diagnosis and evaluation of FAI deformities which further help to differentiate between isolated LT and FAI groups.26
Patient with dysplastic hips were found to have larger labrums, thicker capsules, and larger rectus femoris. Knowing that those soft tissue parameters will differ according to the hip deformities is a first step toward the development of additional preoperative discriminators to improve surgeon's capacity to better categorize borderline hip deformities and permit further optimization of surgical management.
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