The present study demonstrates that subscapularis atrophy may be quantified by measurement of the CSA ratio using a “Y”-view MRI slice, and classified into a four-stage grading system proposed here.
Analyzing atrophy patterns of the subscapularis muscle after reconstruction of isolated subscapularis tendon tears, it has been shown that SSC atrophy always occurs from the top down [15, 28]. The cranial aspect of the SSC seems to be affected the most whereas isolated atrophy within the caudal area of the SSC was not seen in the present cohort nor has been described in the literature previously.
Evaluating the normal MRI images of the 49 control patients, it was seen that the SSC never extends beyond the coracoid process in the oblique sagittal plane. This ventral tip of the scapular “Y” may therefore be considered a bony landmark of the maximal cranial extension of the subscapularis muscle.
While classifications of muscular fatty infiltration are well described by Goutallier et al. [12] by means of computerised tomography (CT) and by Fuchs et al. [9] using MRI, grading systems of muscular atrophy remain rare. Thomazeau et al. [35] and Zanetti et al. [38] published classification systems for supraspinatus atrophy and Warner et al. [37] described a method of classifying combined antero-superior rotator cuff atrophy. In their study Warner et al. [37] introduced a measurement of muscle atrophy based on oblique sagittal plane image medial to coracoid process. The stage is specified by the amount of muscle above or below a line drawn from the edge of the coracoid to the inferior tip of the scapular spine. In addition, the Zanetti tangent line connects the superior aspect of the coracoid and the scapular spine. A convex overhang of the muscle above the line indicates no atrophy and a decrease of the muscle area towards the line or a concavity below the line represents mild, moderate or severe atrophy, respectively. No quantification of atrophy was presented.
Thomazeau et al. [35] used the supraspinatus fossa as osseous landmark to define SSP atrophy. Contrary to the SSP there is no three-side bony limitation of the SSC to guarantee three-side reproducible measurements. Defining osseous landmarks in calculating subscapularis atrophy is challenging and may explain the previous absence of a standardized SSC atrophy grading system.
Schröder et al. [31] and Scheibel et al. [30] semi-quantitatively evaluated atrophy of the subscapularis by measuring the vertical, the cranial-transverse and the caudal-transverse diameters. In addition, the signal to noise ratio of the SSC and the ISP were examined, although without proposing any grading of muscular atrophy. In patients with massive atrophy, usually appearing in the upper part of the muscle, this method may become insufficiently accurate as the cranial-transverse diameter is often difficult to determine due to loss of volume. This potential issue is corroborated by the observations of the present study. Therefore, a reliable determination of atrophy exclusively based on the above mentioned three diameters (vertical, cranial-transverse and caudal-transverse) may be inadequate. Likewise, assessing subscapularis atrophy by comparing the signal to noise ratio of the subscapularis and the infraspinatus muscle may be skewed by the requirement for a non-atrophic infraspinatus muscle.
Atrophy of the subscapularis muscle is commonly seen in the upper part, theorised to be a result of the tendency of the subscapularis tendon to ruptures from the top down, as the inferior part of its humeral insertion is of muscular origin and not tendinous. For this reason, it is useful to evaluate the upper part as compared to the total muscle.
Although isolated caudal lesions of the subscapularis tendon [1, 14, 18] have been, rarely, described in literature, there are no reports of isolated atrophy of the lower part of the SSC. In the present study, caudal atrophy of the SSC was not seen in any of the 11 patients with subscapularis atrophy. It may therefore be assumed that the distal border of the SSC, even in case of higher grades of atrophy, is unlikely to change position significantly.
Given these findings, determining the cross-sectional area seems to be a more reliable method to assess subscapularis atrophy, particularly when using a defined plane in the oblique sagittal “Y-View” (using the most lateral image where the scapular spine is in contact with the body of the scapula). As each patient has a unique subscapularis cross sectional area (atrophic or not), calculating the CSA ratio of the upper part in relation to the total muscle seems to represent a reliable and reproducible tool to assess SSC atrophy.
Using conventional methods, defining the maximal vertical spread from the base to the preserved top of the subscapularis, biased CSA ratios (upper half of the SSC in relation to the total muscle) may result in mild atrophic muscles as compared to severely atrophic cases (see Table 1).
Analyzing the data and MRI images of the control group it was seen that healthy, non-atrophic SSC muscles never extend the coracoid process. Thus, it is proposed to use the ventral tip of the scapular “Y” as a defined osseous landmark for the upper margin of the subscapularis muscle in calculation of the CSA ratio to ensure easily reproducible measurements. This was supported by the good correlation (correlation coefficient: 0.99 - bony referenced) between the two independent examiners determining the CSA ratio in this study.
The major difference between the CSA ratio and the CSA ratio with bony landmark is that the area of the upper half in relation to the total muscle decreases by taking the original vertical diameter for reference (see Fig. 4c and d). Thus, the centerline between the upper and the total muscle is translated cranially and the area of the upper muscle part decreases. In case of (severe) muscle retraction subscapularis measurement with bony landmark may also constitute a more accurate way of evaluation – provided that measurement is performed at a defined sagittal oblique plane of the MRI (“Y-position”). A severe retracted tendon is supposed to lead to a decrease of the CSA ratio, too.
This observation was confirmed by analysis of the measurements of the atrophy group (see Table 1). Compared to the conventional, non-bony-referenced measurement, (mean value 510.0 ± 173.1) the area of the upper part of the SSC was significantly decreased (p < 0.001) in all patients by using the osseous landmark measurement technique (mean value 422.9 ± 173.6). Patients of the atrophy group also had a significantly (p < 0.001) reduced bony landmark CSA ratio with mean values of 0.269 ± 0.065 as compared to the conventionally measured CSA ratio with mean values of 0.322 ± .056. However, it has to be stated that both groups are of limited size. Due to the reason that isolated subscapularis tendon tears only represent a very small part of all rotator cuff tears there is still a lack of studies presenting data regarding isolated subscapularis tears in lager numbers of patients [3, 13, 20, 22] – especially regarding subscapularis atrophy.
Comparing all parameters (three diameters, CSA and CSA ratio) regarding capacity to quantify subscapularis atrophy, the bony referenced CSA ratio represents the most reliable tool with an area under the curve (AUC) of 1.00 (p = .0001).
Based on the bony referenced CSA ratio, a quantitative classification of subscapularis atrophy is proposed as follows:
Stage 0 with a ratio of > 0.4 represents no atrophy and is also seen in healthy patients. Mild atrophy with a CSA ratio < 0.4–0.35 is classified as Stage 1. Stage 2 indicates moderate atrophy with ratio values < 0.35–0.3. Values < 0.3 represent severe atrophy of the subscapularis.
A reliable and reproducible pre-operative analysis of isolated subscapularis atrophy in cases of SSC rupture may assist decision-making and inform choice of treatment and feasibility of subscapularis repair. Postoperatively, it may be used to monitor healing and evaluate clinical outcome.
To confirm this, further prospective combined clinical-radiological studies will be necessary to validate whether quantitative pre-operative assessment of subscapularis atrophy is a useful prognostic indicator of postoperative outcome.