Effect of Tear Classification on Subscapularis Muscle Volume: A Deep Learning-based Semi-automatic Analysis of Pre- and Postoperative Changes in 246 Rotator Cuff Repair Patients With and Without First Facet Subscapularis Tears

Abstract

Background:

Partial-thickness subscapularis (SSC) tendon tears, although less studied than full-thickness tears, are prevalent and clinically significant. Recent classifications have proposed subdivisions for these partial-thickness tears. However, their clinical implications and correlation with SSC muscle volume remain poorly understood.

Purpose:

To evaluate SSC muscle volume in relation to tendon tear classification and assess the changes in muscle volume both pre- and postoperatively using deep learning-based magnetic resonance imaging (MRI) segmentation in patients undergoing arthroscopic rotator cuff repair.

Study Design:

Cohort study; Level of evidence, 3.

Methods:

This study included 246 patients who underwent arthroscopic rotator cuff repair between January 2018 and December 2023. SSC tendon tears were classified using the Yoo and Rhee system. SSC muscle volumes were measured on preoperative MRIs and 6 months postoperatively using a validated deep learning segmentation tool. These volumes were normalized to the scapular volume.

Results:

Preoperative normalized SSC volume (nSSC) varied significantly across tear types (F = 7.21; P < .001). Patients with type 2B tears had a significantly lower mean nSSC (1.45 ± 0.40) than those with no tears (1.68 ± 0.37; P = .018) and type 2A tears (1.74 ± 0.31; P = .023). Six months postoperatively, nSSC significantly decreased in patients with SSC tendon tears (nSSC = 1.48 ± 0.34; ΔnSSC = −0.10 ± 0.24; P < .001). For patients with type 2A or more severe tears, nSSC significantly decreased postoperatively in both the debridement (Δ nSSC = −0.13 ± 0.18; P = .004) and repair (Δ nSSC = −0.12 ± 0.29; P = .017) groups. However, the degree of volume change was not significantly different between the treatment groups.

Conclusion:

Even in partial-thickness tears affecting only the first facet, nSSC muscle volume decreases significantly with increasing tear severity. Furthermore, SSC muscle volume significantly decreases 6 months postoperatively in patients with SSC tendon tears, although the extent of this reduction does not vary among different treatment modalities.

Keywords

deep learning muscle volume segmentation shoulder subscapularis tendon tear

The subscapularis (SSC) is the largest and most powerful muscle of the rotator cuff, serving as a primary internal rotator of the shoulder and contributing to the transverse force couple essential for shoulder stability.^7,19 Historically, SSC tendon tears have been underdiagnosed; however, advancements in imaging and arthroscopic techniques have enhanced their clinical recognition.^18,22 These tears may occur in isolation or more commonly in conjunction with posterosuperior rotator cuff tears.²⁷

Partial-thickness SSC tendon tears, especially those affecting the first facet, have been less emphasized than full-thickness tears, despite their common occurrence and potential clinical significance.¹⁸ Yoo et al²⁷ have developed a classification system that further categorizes Lafosse type 1 partial-thickness tears into type 2A (<50% detachment of the first facet) and type 2B (>50% detachment of the first facet), highlighting the need to stratify first facet partial-thickness SSC tendon tears. However, the clinical relevance of this classification and its implications for treatment remain poorly understood, with few studies investigating the outcomes of these partial tears.⁸

Previous research has demonstrated that with increasing tear severity, SSC muscle atrophy and fatty degeneration progress, ultimately leading to functional impairments that necessitate surgical repair for full-thickness SSC tendon tears.^10,15 However, volumetric changes in the SSC muscle associated with partial-thickness tears remain poorly understood. The advent of deep learning-based automated magnetic resonance imaging (MRI) segmentation tools has revolutionized the analysis of rotator cuff muscle volume across larger patient cohorts, addressing previous limitations because of small sample sizes and the dependency on manual measurements.^3,20 This technological advancement allows for the comparison of baseline rotator cuff muscle volumes across a broader population, extending beyond the confines of longitudinal assessments within individual patients.^1,17,25,26

This study aimed to (1) investigate preoperative SSC muscle volume according to SSC tendon tear classification and (2) evaluate pre- and postoperative changes in SSC muscle volume in a large cohort of patients undergoing arthroscopic rotator cuff repair. We hypothesized that (1) SSC muscle volume would significantly decrease with increasing tear severity and (2) postoperative SSC muscle volume would further decline in patients with SSC tendon tears.

Methods

This retrospective cohort study was conducted at a single tertiary hospital and approved by the institutional review board (IRB) of Korea University Medical Center (IRB approval no. 2023AN0559).

Cohort Selection

This study analyzed patients who underwent arthroscopic rotator cuff repair between January 2018 and December 2023. All procedures were conducted by a single senior surgeon (W.K.J.), with >15 years of experience in shoulder and elbow surgery. The inclusion criteria were patients who underwent both pre- and postoperative MRI scans conducted at our institution. The exclusion criteria included revision surgeries, follow-up periods of <1 year, inadequate MRI quality for analysis, irreparable posterosuperior rotator cuff tears, and tendon transfers (Figure 1). Among the 246 enrolled patients, 9 had isolated SSC tendon tears, while the remainder underwent surgery for posterosuperior rotator cuff tears, either with or without concomitant SSC tendon tears.

Figure 1.

Cohort selection. ISP, infraspinatus; MRI, magnetic resonance imaging; SSP, supraspinatus.

Data Collection

Comprehensive reviews of medical records and operative notes were conducted for all eligible patients. Patient characteristics included age, sex, laterality, body mass index, follow-up duration, and the presence of diabetes mellitus, dyslipidemia, and a history of steroid injections. Work activity levels were assessed using initial patient assessment forms and categorized into 3 levels, as previously described.¹³

Arthroscopic findings were obtained by reviewing the operative notes for all enrolled patients. Throughout the study period, operative notes were initially drafted by a senior resident immediately after each procedure and were subsequently verified by the operating surgeon on the same day. To ensure data completeness and accuracy, archived full-length arthroscopic videos were reviewed to supplement any missing details from the operative notes. The key variables collected included primary diagnosis, SSC tendon tear type, tear size of the SSP and infraspinatus (ISP) (in mm), tear classification according to Cofield et al,² biceps pathology, and procedures performed for SSC tendon tears, including debridement or repair.

Surgical Procedures and Rehabilitation

Routine diagnostic arthroscopy was performed on all patients to assess the status of the SSC tendon. SSC tendon tears were classified using the Yoo and Rhee system²⁷ as follows: type 1, fraying or longitudinal splitting of the leading edge of the SSC tendon; type 2A, detachment of <50% of the tendon from the first facet; type 2B, detachment of >50% from the first facet; type 3, a full-thickness tear of the entire first facet; type 4, exposure of both the first and second facets with medial retraction of the tendon; and type 5, a complete tear involving the muscular portion. SSC tendon tears were identified during intra-articular examination through the posterior glenohumeral portal. Type 3 tears were easily visualized as full-thickness detachments of the SSC tendon from its footprint. In contrast, type 2B and lower-grade tears were identified by placing the arm in internal rotation and 45° abduction while applying a posteriorly directed force, which allowed visualization of the first facet of the SSC footprint and assessment of tendon detachment at that site (Figure 2). Concealed SSC tendon tears were identified by opening the rotator interval from the subacromial space. This step was performed only when no abnormalities were observed during intra-articular examination, but an SSC tear was suspected based on pre- or intraoperative findings. Suspicion of a concealed tear was raised in the presence of 1 or more of the following: (1) biceps tendon tear or subluxation on preoperative MRI; (2) a positive bear hug test on the physical examination; or (3) significant biceps instability detected with a probe during intra-articular arthroscopic assessment.

Figure 2.

Arthroscopic photographs of the type 1, 2A, and 2B SSC tendon tears in the intra-articular space viewed from the posterior glenohumeral portal. (A) Yoo and Rhee type 1 tear showing fraying (arrows) of the SSC tendon without detachment of the first facet. (B) Type 2A tear with <50% detachment from the first facet (solid outline). (C) Type 2B tear with >50% detachment of the first facet (solid outline) and partial disruption of the lateral hood (red solid line). SSC, subscapularis.

Repairs were performed on all shoulders with type 2B or higher SSC tendon tears. For patients with type 2A tears, repairs were conducted if they exhibited a positive bear hug test during the preoperative physical examination. Types 1 and 2A tears without a positive bear-hug test were treated by debridement of the frayed tissue. Type 2B and 2A tears were repaired using a single-row suture technique within the intra-articular space, whereas type 3 tears were repaired in the subacromial space using the same technique. Biceps pathologies were managed according to the severity of tendon injury. Tendons with surface fraying were treated with debridement, whereas partial or complete ruptures were managed with biceps tenotomy followed by tenodesis. Biceps tenodesis was performed arthroscopically using an additional suture anchor placed in the bicipital groove, and the tendon was firmly tied down from the subacromial space. For SSP or ISP tendon tears, which constituted the primary diagnosis in most patients in this study, except for 9 cases with isolated SSC tendon tears, the repair was performed using either a transosseous-equivalent technique or single-row repair in the subacromial space, depending on the tear configuration.

All patients were fitted with abduction braces and adhered to a standardized postoperative rehabilitation protocol. The protocol began with immobilization for the first 6 weeks after surgery. Passive range of motion (ROM) exercises were commenced thereafter. Once full passive ROM was achieved, active-assisted ROM exercises were introduced. Strengthening exercises began 12 weeks postoperatively and continued for approximately 6 months.

MRI Protocol and Volume Measurement

All shoulder MRIs were performed preoperatively and 6 months postoperatively using a 3.0-T MRI scanner with a dedicated shoulder coil. Postoperative MRIs were obtained at 6 months to evaluate the integrity of the repaired rotator cuff tendon before patients returned to more demanding physical activity. Scans were performed on one of the following scanners: Magnetom TrioTrim, Skyra, and Prisma (Siemens) or Achieva (Philips). The imaging protocol included coronal oblique, sagittal oblique, and axial planes with a slice thickness of 2.5 mm and a field of view of 16 × 16 cm. T1 oblique sagittal images extended from the most lateral edge of the rotator cuff tendon footprints to 2 cm medial to the Y-view (Y+2 view).¹ The imaging protocol and coverage range for the oblique sagittal slices were consistently maintained throughout the study. All MRI data were stored in Digital Imaging and Communications in Medicine format and were anonymized to ensure patient confidentiality.

For volume analysis, a previously validated automated MRI segmentation software was used to segment all 4 rotator cuff muscles and the scapula bone (MEDIP PRO Version 2.0.0; MEDICAL IP Co).¹¹ This software is based on a 3-dimensional (3D) nnU-Net architecture, which is a recognized model for medical image segmentation.^16,21 The software initially generated segmentation outlines, which were subsequently reviewed and confirmed by 2 fellowship-trained orthopaedic surgeons (K.S.H. and J.H.L.). Any erroneous segmentation outlines were manually corrected. The segmented outlines were reconstructed into 3D images of the SSC muscle and scapula (Figure 3). The corresponding muscle and bone volumes were calculated in cubic centimeters (cm³;). The normalized SSC muscle volume (nSSC) was calculated by dividing the SSC volume by the scapula bone volume, using a previously validated method for normalizing rotator cuff muscle volumes.^23,24

Figure 3.

(A) Representative T1-weighted oblique sagittal image demonstrating the segmentation of the SSP (green), ISP (pink), SSC (blue), teres minor (violet), deltoid (brown), and scapula (white). (B) 3-dimensional reconstruction of the SSC muscle (blue) and scapula bone (white). ISP, infraspinatus; SSC, subscapularis; SSP, supraspinatus.

Statistical Analysis

A priori power analysis was performed to determine the sample size required to achieve 80% power. This analysis assumed an allocation ratio of 12 to 10 to 4 to 4 to 1 (No tear: type 1 tear: type 2A tear: type 2B tear: type 3 tear) across tear types, with an effect size of 0.3 and an alpha level of .05, to detect differences in the distribution of sex across tear classifications. The allocation ratio was derived from data from our institutional registry of patients undergoing arthroscopic rotator cuff surgery. Tear types classified as type ≥4 were excluded from the analysis because most of these cases were managed with open surgery at our institution. Power analysis indicated that a minimum sample size of 133 patients was required.

Continuous variables were compared among the tear types using analysis of variance (ANOVA). For variables showing significant differences among groups, post-hoc pairwise comparisons were performed using the Bonferroni test. Categorical data were analyzed using the chi-square test or the Fisher exact test, as appropriate. Pre- and postoperative nSSC were compared using paired t tests. Statistical significance was set at P < .05. All analyses were conducted using R software Version 4.4.0 (R Core Team).

Results

Patient Characteristics

A total of 246 patients were enrolled, with a mean age of 61.88 ± 9.95 years, including 108 men (43.90%) (Table 1). Among these, 136 (55.28%) patients had SSC tendon tears, including 9 (3.66%) with isolated SSC tendon tears (Table 2). The distribution of SSC tendon tear types was as follows: type 1, 74 (30.08%); type 2A, 26 (10.57%); type 2B, 29 (11.79%); and type 3, 7 (2.85%). Of those with SSC tendon tears, 97 (39.43%) underwent debridement and 39 (15.85%) underwent repair. In addition, 105 patients (42.68%) had concomitant biceps pathology.

Table 1

Patient Characteristics ^a

Variables	Mean ± SD or N (%)
N	246 (100)
Age, years	61.88 ± 9.95
Sex: Male	108 (43.90)
Affected side: right	171 (69.51)
Affected side: dominant	192 (78.05)
Symptom duration, months	15.77 ± 20.26
BMI, kg/m²	25.15 ± 3.05
Work activity: low/medium/high	140/47/59 (56.91/19.11/23.98)
DM	40 (16.26)
DL	49 (19.92)
No. of steroid injections	0.59 ± 1.65
Follow-up, months	18.15 ± 6.83

BMI, body mass index; DL, dyslipidemia; DM, diabetes mellitus.

Table 2

Arthroscopic Findings ^a

Arthroscopy findings	N (%)
Main diagnosis
PTRCT	48 (19.51)
FTRCT: small/medium/large	45/117/27 (18.29/47.56/10.98)
Isolated SSC tear	9 (3.66)
SSC tear type, Yoo and Rhee
No tear	110 (44.72)
1	74 (30.08)
2A	26 (10.57)
2B	29 (11.79)
3	7 (2.85)
SSC procedure: debridement/repair	97/39 (39.43/15.85)
Biceps pathology
Normal	141 (57.32)
Fraying	36 (14.63)
Partial or complete tear	59 (23.98)
Subluxation	4 (1.63)
Absent	1 (0.41)

FTRCT, full-thickness rotator cuff tear; PTRCT, partial-thickness rotator cuff tear; SSC, subscapularis.

Preoperative nSSC Volume

ANOVA revealed a significant difference in the mean nSSC across tear classifications (F = 7.21; P < .001). Mean values were as follows: Normal, 1.68 ± 0.37; type 1, 1.63 ± 0.31; type 2A, 1.74 ± 0.31; type 2B, 1.45 ± 0.40; and type 3, 1.10 ± 0.22 (Table 3 and Figure 4). Post-hoc pairwise comparisons using the Bonferroni test demonstrated significant differences between the type 2B and normal groups (P = .018), as well as between type 2B and type 2A (P = .023). Type 3 also showed significant differences compared with the normal (P < .001), type 1 (P = .001), and type 2A (P = .002) groups (Figure 4). No significant differences were observed among the normal, type 1, and type 2A groups. No significant differences were observed in the distribution of sex (χ²; = 3.50; P = .478), retraction of SSP/ISP tears (F = 0.982; P = .418), or age groups (χ²; = 27.75; P = .115) across tear classifications.

Table 3

Preoperative Mean nSCC According to Tear Types ^a

	SSC Tendon Tear Type, Yoo and Rhee
Variables	No tear	Type 1	Type 2A	Type 2B	Type 3	F or χ²;	P value ^b
N (%)	110 (44.72)	74 (30.08)	26 (10.57)	29 (11.79)	7 (2.85)
nSSC	1.68 ± 0.37	1.63 ± 0.31	1.74 ± 0.31	1.45 ± 0.40	1.10 ± 0.22	7.21	<.001
Sex: Male	47 (42.73)	29 (39.19)	13 (50)	14 (48.28)	5 (71.43)	3.50	.478
SSP/ISP retraction, mm	14.4 ± 8.93	15.6 ± 7.84	16.5 ± 10.1	17.7 ± 12.3	17.9 ± 11.5	0.982	.418
Age group, years						27.75	.115
15-29	2 (1.82)	1 (1.35)	0 (0)	0 (0)	0 (0)
30-39	1 (0.91)	1 (1.35)	0 (0)	0 (0)	0 (0)
40-49	16 (14.55)	6 (8.11)	0 (0)	1 (3.45)	1 (14.29)
50-59	28 (25.45)	12 (16.22)	5 (19.23)	6 (20.69)	0 (0)
60-69	51 (46.36)	34 (45.95)	11 (42.31)	13 (44.83)	6 (85.71)
70-89	12 (10.91)	20 (27.03)	10 (38.46)	9 (31.03)	0 (0)

Data are presented as n (%) or mean ± SD. The bold P value indicates statistical significance. ISP, infraspinatus; nSSC, normalized subscapularis volume; SSC, subscapularis; SSP, supraspinatus.

Analysis of variance or χ²; test.

Figure 4.

The mean (diamonds) and standard deviation (error bars) of the nSSC volume calculated as the ratio of the SSC muscle volume to the scapular bone volume according to the tear type. *P < .05; **P < .01; ***P < .001. nSSC, normalized subscapularis.

Normalized SSC Volume 6 Months Postoperatively

Mean nSSC decreased significantly at 6 months postoperatively in patients with SSC tendon tears (nSSC = 1.48 ± 0.34; ΔnSSC = −0.10 ± 0.24; P < .001) (Table 4 and Figure 5). In contrast, no significant decrease was noted in mean postoperative nSSC in patients with an intact SSC tendon (nSSC = 1.63 ± 0.48; ΔnSSC = − 0.05 ± 0.47; P = .240).

Table 4

Volume Change 6 Months Postoperatively ^a

SSC Tendon	nSSCPostoperatively 6 Mo.	ΔnSSC ^b	P
No tear	1.63 ± 0.48	−0.05 ± 0.47	.240
Tear	1.48 ± 0.34	−0.10 ± 0.24	<.001
Type 1	1.54 ± 0.32	−0.09 ± 0.23	.001
Type 2A	1.62 ± 0.32	−0.12 ± 0.20	.004
Type 2B	1.34 ± 0.28	−0.11 ± 0.31	.059
Type 3	0.98 ± 0.27	−0.12 ± 0.16	.093

Data are presented as mean ± SD. nSSC, normalized subscapularis volume; Postop, postoperative; Preop, preoperative; SSC, subscapularis.

ΔnSSC = postop nSSC – preop nSSC.

Figure 5.

Preoperative (black) and 6-month postoperative (red) nSSC, calculated as the ratio of the SSC muscle volume to the scapular bone volume according to tear type. The data are presented as mean values (diamonds) and standard deviations (error bars).

In the subgroup analysis of patients with Yoo and Rhee type 2A or higher tears, regardless of whether debridement or repair was performed, nSSC significantly decreased at 6 months postoperatively in both the debridement group (nSSC = 1.54 ± 0.37; ΔnSSC = −0.13 ± 0.18; P = .004) and the repair group (nSSC = 1.34 ± 0.33; ΔnSSC = −0.12 ± 0.29; P = .017) (Table 5). However, no significant difference was observed in the change of nSSC (P = .862) between the 2 treatments.

Table 5

SSC Muscle Volume Change According to Treatment in Type 2 and More SSC Tendon Tears ^a

Treatment	nSSCPostoperatively 6 Mo.	ΔnSSC ^b	P
Debridement, n = 23	1.54 ± 0.37	−0.13 ± 0.18	.004
Repair, n = 39	1.34 ± 0.33	−0.12 ± 0.29	.017
P		.862

Data are presented as mean ± SD. nSSC, normalized subscapularis volume; Postop, postoperative; Preop, preoperative; SSC, subscapularis.

ΔnSSC = postop nSSC – preop nSSC.

Discussion

To the best of our knowledge, this is the first study to analyze both pre- and postoperative SSC muscle volumes in a large cohort of patients with rotator cuff tears. Volumetric analysis of this extensive dataset revealed significant differences in the mean nSSC among various types of SSC tendon tears. Notably, the mean nSSC was significantly lower in patients with Yoo and Rhee type 2B tears than in those with no SSC tendon tear or type 2A tears. These findings underscore that, beyond the well-documented link between advanced SSC tears and muscle atrophy, partial-thickness tears affecting only the first facet can also lead to progressive muscle atrophy.

While MRI segmentation is a common method for measuring rotator cuff muscle volumes in clinical research, most previous studies have concentrated on longitudinal changes in the supraspinatus muscle volume.^1,17,25,26 These investigations, limited by small sample sizes ranging from 31 to 95 patients, likely lacked sufficient statistical power to effectively compare baseline rotator cuff muscle volumes. Moreover, studies analyzing SSC muscle volume in patients with rotator cuff tears are even rarer. To the best of our knowledge, only 1 such study exists; however, it did not categorize SSC tear types in detail, thereby limiting the scope of its analysis.⁹ A recent study by Riem et al²⁰ utilized artificial intelligence-based automated MRI segmentation software to evaluate rotator cuff muscle volumes in a larger cohort of 170 patients. However, that study only included individuals with intact rotator cuff tendons. In contrast, our study examined the pre- and postoperative SSC muscle volumes in 246 patients, totaling 492 MRIs, making it the largest cohort to date, to our knowledge. This scale allowed for a detailed comparison of SSC muscle volumes across different SSC tendon tear severities.

Various authors have proposed different classifications for SSC tendon tears.⁵ Among these, the Lafosse classification¹⁴ is the most widely accepted and commonly used. It categorizes SSC tendon tears into upper one-third partial tears, complete upper one-third tears, complete upper two-thirds tears, and complete tears with or without tendon degeneration.¹⁴ Yoo et al²⁷ reported that the SSC footprint comprises 4 distinct facets, with the first facet accounting for approximately one-third of the entire footprint, based on a 3-dimensional analysis of 39 fresh cadaveric specimens. The authors noted that partial tears confined to the first facet corresponded to Lafosse type 1 tears and proposed a further subdivision into types 2A and 2B within their classification system. This classification is clinically significant as it further delineates partial-thickness tears, which are most commonly encountered during arthroscopic rotator cuff repair but have traditionally received less attention than full-thickness tears. However, a consensus on the long-term prognosis and treatment strategies for type 2A and type 2B tears has yet to be established.^4,8,18 The findings of this study, showing a reduction in SSC muscle volume in partial-thickness SSC tendon tears, provide an additional rationale for the subclassification of these tears, which were traditionally grouped as Lafosse type 1 tears.

The SSC muscle volume at 6 months postoperatively decreased significantly in patients with SSC tendon tears, whereas no significant change was observed in those with intact SSC tendons, supporting our second hypothesis. Notably, the mean nSSC in patients with intact SSC tendons showed approximately half of the reduction observed in those with tendon tears, although this change was not statistically significant. This decline may be attributed to the initial 6-week immobilization period, followed by delayed initiation of strengthening exercises 3 months postoperatively. We compared postoperative SSC muscle volume changes between patients who underwent debridement and those who underwent repair for type 2A or higher SSC tendon tears. Contrary to our expectation that postoperative SSC muscle volume would decrease more in patients who underwent debridement than in those who underwent repair, no significant difference in nSSC was observed between the 2 groups. These findings may appear inconsistent with a previous level 1 clinical trial that demonstrated better strength in patients with type 2B SSC tendon tears who underwent repair than in those who underwent debridement.⁸ However, that study assessed outcomes, including clinical scores and strength, over a follow-up period exceeding 5 years. In contrast, the 6-month follow-up MRI in our study may have been too brief to detect significant differences in muscle volume changes, especially considering that the patients were allowed to begin active shoulder exercises only 3 months postoperatively.

The optimal timing for follow-up MRI to detect meaningful recovery of this rotator cuff muscle after surgery remains unclear. Hamano et al⁶ reported recovery of SSP muscle atrophy and fatty infiltration as early as 12 months after arthroscopic rotator cuff repair. However, muscle volume was estimated using 2D surface area measurements from a single sagittal slice.⁶ Similarly, Xu et al²⁶ demonstrated significant 3D SSP muscle volume recovery at 12 months compared with that at 3 months postoperatively. However, the mean volume at 12 months showed a slight decrease relative to preoperative values, with no significant improvement. A more recent study by Kriechling et al,¹² a follow-up to their earlier investigation of 3D SSP volume after successful versus failed repairs,²⁵ reported progressive muscle recovery 12 to 60 months postoperatively. At 12 months, significant improvements were observed only when compared with the 3-month postoperative volume, with no significant difference from preoperative measurements. However, by 60 months, the SSP muscle volume had significantly improved relative to baseline, indicating true long-term recovery. Based on these findings, a minimum follow-up interval of 1 year is advisable, with optimal assessment likely requiring MRI at 2 to 5 years postoperatively to capture definitive recovery. In our cohort, postoperative MRI scans were obtained at 6 months to confirm tendon integrity before the resumption of strenuous activity. Although this short-term follow-up was clinically practical, long-term imaging will likely be required to assess the true course of muscle volume recovery and its possible dependence on different treatment strategies.

The mean preoperative nSSC in type 1 tears was slightly lower than that in type 2A tears, although the difference was not statistically significant. This finding may appear counterintuitive, considering our primary hypothesis that greater tear severity is associated with reduced preoperative muscle volume. A possible explanation for this observation is the potential presence of concealed lesions within the type 1 tear group.²⁷ Type 1 tears were assessed during routine intra-articular arthroscopic examination, whereas more subtle type 2 tears typically required identification under internal rotation with 45° abduction of the arm. Concealed type 2A or 2B tears may have gone undetected in patients classified as having type 1 tears in our study. Such lesions are only revealed after opening the rotator interval from the subacromial side of the shoulder, which is not routinely performed. In our cohort, concealed lesions, all of which were classified as type 2A tears, were identified in 3 patients. These cases were suspected preoperatively based on clinical and imaging findings. However, in patients who exhibited only minimal signal changes around the SSC footprint on imaging and lacked biceps pathology or physical examination findings suggestive of SSC involvement, the rotator interval was not opened if no abnormalities were observed during intra-articular arthroscopic assessment. Therefore, some concealed lesions may have been undetected during surgery, potentially contributing to a relatively lower nSSC in the type 1 group.

Although variability in the coverage range of oblique sagittal MRI series with partial scapula coverage could be a concern, most MRI scans in our study were acquired using a consistent imaging protocol, ensuring uniform coverage across all patients. Moreover, the nSSC muscle volume was calculated as the ratio of SSC muscle volume to scapular bone volume captured within the oblique sagittal series, effectively neutralizing the effect of varying coverage ranges. In contrast to concerns regarding medial retraction affecting the captured slices, all patients in our study had Yoo and Rhee type 3 or lower SSC tendon tears, indicating that medial tendon retraction was minimal across the cohort, thereby mitigating this potential source of bias.

Limitations

Our study had a few limitations. First, although a consistent MRI protocol and standardized scapula coverage ensured reliable comparisons within our cohort, the interpretation of nSSC muscle volume data should be approached with caution, particularly when comparing our results to those of other studies. Variations in scapula coverage across different studies may lead to discrepancies in reported values. Second, Dixon MRI was not routinely performed; therefore, the intramuscular fat proportion could not be quantified. Additional analysis reflecting fat infiltration may enhance the depth of our evaluation. Third, our study did not include an analysis of type ≥4 tears, as these were treated with open repairs at our institution. However, our primary focus was on partial-thickness tears involving the first facet of the SSC footprint. Studies incorporating volume analysis of more advanced SSC tendon tears would help strengthen and broaden the implications of our findings. Fourth, the 6-month postoperative follow-up MRIs in our study may have been insufficient to detect meaningful differences in SSC volume changes among the different treatments. Future studies with longer-term follow-up MRIs are needed to identify significant differences between the treatment groups. Finally, although the SSC footprint was carefully assessed by a single experienced senior surgeon (W.K.J.), the distinction between type 2A and type 2B tears can be challenging in some cases and may introduce an element of subjectivity. However, all surgical records were independently drafted by a senior resident who assisted in the procedures and were subsequently reviewed and confirmed by the operating surgeon on the same day of surgery. We believe that this process helped to minimize the potential classification bias.

Conclusion

The preoperative nSSC muscle volume significantly decreased with increasing tear severity, including partial-thickness tears that involved only the first facet. SSC muscle volume significantly decreased 6 months postoperatively in patients with torn SSC tendons, regardless of the treatment method.

Footnotes

Final revision submitted May 28, 2025; accepted June 24, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: This research was supported by a grant from Korea University Anam Hospital, Seoul, Republic of Korea (02514271). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from the Institutional Review Board (IRB) of Korea University Medical Center (IRB approval No. 2023AN0559).

ORCID iDs

Kyosun Hwang

Jin Hyeok Lee

Dongkyun Lim

Kanghun Yu

Woong Kyo Jeong

References

Chung

Moon

et al. Serial changes in 3-dimensional supraspinatus muscle volume after rotator cuff repair. Am J Sports Med. 2017;45(10):2345-2354.

Cofield

Parvizi

Hoffmeyer

Lanzer

Ilstrup

Rowland

CM.

Surgical repair of chronic rotator cuff tears. A prospective long-term study. J Bone Joint Surg Am. 2001;83(1):71-77.

DeFoor

Riem

Cognetti

et al. Novel 3D MRI-based volumetric assessment of rotator cuff musculature demonstrates stronger correlation with preoperative functional status when compared to the Goutallier grading scheme. J Shoulder Elbow Surg. 2024;33(11):e575-e584.

Garg

Meena

Farinelli

D'Ambrosi

Tapasvi

Braun

Partial subscapularis tear: State-of-the-art. J ISAKOS. 2024;9(6):100290.

Ghasemi

McCahon

JAS

Yoo

et al. Subscapularis tear classification implications regarding treatment and outcomes: consensus decision-making. JSES Rev Rep Tech. 2023;3(2):201-208.

Hamano

Yamamoto

Shitara

et al. Does successful rotator cuff repair improve muscle atrophy and fatty infiltration of the rotator cuff? A retrospective magnetic resonance imaging study performed shortly after surgery as a reference. J Shoulder Elbow Surg. 2017;26(6):967-974.

Iagulli

Field

Hobgood

Ramsey

Savoie

III . Comparison of partial versus complete arthroscopic repair of massive rotator cuff tears. Am J Sports Med. 2012;40(5):1022-1026.

Jeong

Kim

Lee

Yoo

JC.

Prospective randomized clinical trial of arthroscopic repair versus debridement for partial subscapularis tendon tears more than half of the entire first facet. Am J Sports Med. 2023;51(11):2804-2814.

Jun

Moon

Bhardwaj

Lim

Cha

DH.

The volume of subscapularis muscle remains unaffected by supraspinatus tendon tears: three-dimensionally reconstructed magnetic resonance imaging analysis. Clin Shoulder Elb. 2019;22(1):3-8.

10.

Kilic

Ardebol

Pak

Menendez

Denard

PJ.

Higher upper subscapularis Goutallier grade and coracohumeral distance narrowing are predictive of subscapularis tears in patients undergoing arthroscopic rotator cuff repair. Arthroscopy. 2024;40(5):1397-1406.

11.

Kim

Yoo

Yoon

et al. Development of a deep learning-based fully automated segmentation of rotator cuff muscles from clinical MR scans. Acta Radiol. 2024;65(9):1126-1132.

12.

Kriechling

Joshy

Klotz

et al. 3D muscle volume and 3D fat fraction after successful and failed arthroscopic rotator cuff repair at 5-year follow-up. Am J Sports Med. 2025;53(3):571-582.

13.

Kwon

Kim

Lee

Kim

JH.

The rotator cuff healing index: a new scoring system to predict rotator cuff healing after surgical repair. Am J Sports Med. 2019;47(1):173-180.

14.

Lafosse

Lanz

Saintmard

Campens

Arthroscopic repair of subscapularis tear: surgical technique and results. Orthop Traumatol Surg Res. 2010;96(suppl 8):S99-S108.

15.

Maman

Harris

White

Tomlinson

Shashank

Boynton

Outcome of nonoperative treatment of symptomatic rotator cuff tears monitored by magnetic resonance imaging. J Bone Joint Surg Am. 2009;91(8):1898-1906.

16.

Medina

Buckless

Thomasson

Torriani

Deep learning method for segmentation of rotator cuff muscles on MR images. Skeletal Radiol. 2021;50(4):683-692.

17.

Nagawa

Hara

Shimizu

et al. Three-dimensional sectional measurement approach for serial volume changes in shoulder muscles after arthroscopic rotator cuff repair. Eur J Radiol Open. 2024;13:100577.

18.

Ono

Sakai

Carroll

IKY

. Tears of the subscapularis tendon: a critical analysis review. JBJS Rev. 2017;5(3):e1.

19.

Richards

Burkhart

Campbell

SE.

Relation between narrowed coracohumeral distance and subscapularis tears. Arthroscopy. 2005;21(10):1223-1228.

20.

Riem

Blemker

DuCharme

et al. Objective analysis of partial three-dimensional rotator cuff muscle volume and fat infiltration across ages and sex from clinical MRI scans. Sci Rep. 2023;13(1):14345.

21.

Ronneberger

Fischer

Brox

. U-net: Convolutional networks for biomedical image segmentation. In: Medical image computing and computer-assisted intervention–MICCAI 2015: 18th international conference, part III 18. Springer; 2015:234-241.

22.

Ticker

Burkhart

SS.

Why repair the subscapularis? A logical rationale. Arthroscopy. 2011;27(8):1123-1128.

23.

Werthel

Boux

Casson

Burdin

et al. CT-based volumetric assessment of rotator cuff muscle in shoulder arthroplasty preoperative planning. Bone Jt Open. 2021;2(7):552-561.

24.

Werthel

Boux

Casson

Walch

et al. Three-dimensional muscle loss assessment: a novel computed tomography-based quantitative method to evaluate rotator cuff muscle fatty infiltration. J Shoulder Elbow Surg. 2022;31(1):165-174.

25.

Wieser

Joshy

Filli

et al. Changes of supraspinatus muscle volume and fat fraction after successful or failed arthroscopic rotator cuff repair. Am J Sports Med. 2019;47(13):3080-3088.

26.

Liu

Qiao

Zhao

Longitudinal changes in overall 3D supraspinatus muscle volume and intramuscular fatty infiltration after arthroscopic rotator cuff repair. J Bone Joint Surg Am. 2024;106(3):218-226.

27.

Yoo

Rhee

Shin

et al. Subscapularis tendon tear classification based on 3-dimensional anatomic footprint: a cadaveric and prospective clinical observational study. Arthroscopy. 2015;31(1):19-28.