The main findings of this study were that 1) there was positive correlation between the total joint volume and each shoulder motion and 2) there were positive correlations between the posterior half compartment joint volume, inferior half compartment joint volume and each shoulder motion.
Postoperative stiffness after ARCR is commonly reported morbidity after ARCR and concerns to shoulder surgeons due to inferior functional outcome when developed.[1, 2, 6] Although the pathophysiology of stiffness after ARCR is not well understood, postoperative ROM is affected by several factors, such as capsular contracture, contracture or atrophy of the rotator cuff itself, and adhesions within the extra-articular glenohumeral motion interface.[11, 14] Several investigators tried to quantitatively measure pathologic regions in MRI images for evaluating capsular contracture; the width, depth, and height of the axillary recess, dimension of rotator interval and the glenohumeral distance.[20-22] However, these studies evaluated pathologic regions in 2D images, which may not accurately reflect the status of the capsular contracture. Other authors measured the capsular volume of the glenohumeral joint according to the volume of fluid injected into the capsule with or without pressure measurement.[1, 11, 23] Even though these methods are excellent for evaluating capsular contracture, procedures such as fluid injection, volume and/or pressure measurement are not easily applied to patients during follow-up after operation. In the current study, we calculated the volume of the shoulder joint using CTA. CTA has been routinely performed to patients who underwent ARCR at postoperative 6 months in our hospital for evaluating cuff continuity, and selection bias can be reduced. Using 3D Slicer software which has been used in a variety of medical studies,[15, 17, 18] and its accuracy and efficiency in 3-dimensional segmentation and analysis have been well described, we could measure capsular volume through CTA DICOM files.
Decreased total joint volume in a primary stiff shoulder is well documented in previous studies.[22, 24, 25] However, previous studies used comparative analyses and reported decreased joint volume in the stiff group. Therefore, the relation between joint volume and ROM could not be evaluated. Moreover, the subjects in the previous study were primary stiff shoulder patients. Primary shoulder stiffness and postoperative stiffness after ARCR are considered to be different disease entities due to their different natural courses,[6, 8, 18, 26, 27] therefore, the results of the previous studies could not be applied to postoperative stiffness after ARCR. In the current study, we performed a correlation analysis between total joint volume and postoperative ROM after ARCR. The correlation analysis between total joint volume and each shoulder motion showed a moderately positive correlation with each shoulder motion (Sc: Pearson coefficient, 0.32, p =0.0047; ERs: Pearson coefficient, 0.24, p=0.0296; ER90: Pearson coefficient, 0.33, p=0.0023; IRb: Pearson coefficient, 0.23, p=0.0336). This result implies that total joint volume is related to the postoperative ROM and that a decrease in total joint volume could lead to postoperative stiffness. Therefore, procedures to increase the total joint volume are crucial to increase the postoperative ROM, and this can further lead to the prevention of postoperative stiffness. Although the usefulness of capsulectomy and early rehabilitation remain controversial,[1, 6, 8, 13] based on our results, procedures that increase total joint volume may help prevent postoperative stiffness from a clinical perspective.
In the current study, we could easily separate specific areas of joint volume (anterior half, posterior half, superior half and inferior half) using 3D Slicer software and evaluate the relation between specific area joint volume and ROM after ARCR. Our study showed that anterior half compartment joint volume was related to ERs, posterior half compartment joint volume related to IRb, and inferior half compartment joint volume related to Sc. These findings were similar to the results of previous studies,[10, 28-30] but different findings were that posterior half and inferior half compartment joint volume were related to all shoulder motion; not only the IRb and Sc but also the external rotation motions (ERs and ER90). Even though we cannot delineate the cause of these findings from current study, positive correlations between posterior half and inferior half compartment joint volume and all shoulder motion are clinically meaningful. In the evidence base review (Level V study) by Itoi et al,[14] they suggested that shoulder stiffness after rotator cuff surgery is typically global, but posterior capsular contracture is often accentuated. However, they did not suggest clinical relevance of posterior capsular contracture. Given that the posterior capsular contracture is often accentuated after ARCR and posterior half and inferior half compartment joint volume are related to all shoulder ROM,[14] efforts to increase the posterior half compartment joint volume and inferior half compartment joint volume such as selective capsulectomy during operation and posterior capsular stretching exercise after operation may be useful in the prevention and treatment of postoperative stiffness.
There are several limitations in the current study. (1) Among 1318 consecutive patients during the study period, 242 patients were included after applying the inclusion criteria. During applying inclusion criteria, only 18% patients were selected. In addition, among 242 patients, 83 patients were selected as final subjects after applying the exclusion criteria. Thus, this study may have a selection bias. However, we wanted to evaluate the relation between joint volume and ROM after arthroscopic repair of small to medium sized rotator cuff tears. In addition, we also excluded patients who had factors that are known to affect postoperative stiffness. Considering that the final subjects are selected after applying inclusion and exclusion criteria among consecutive patients, selection bias may be minimized. (2) Evaluation of the patients' ROM was done at 6 months postoperatively, which can be a short observation period for defining postoperative stiffness. However, “naturally resolving” postoperative stiffness patients mostly shows improvement of ROM within 6 months and further observation period makes no difference because tendon stops healing and starts to remodel at this time.[1, 19, 27, 28] (3) Although we obtained excellent interobserver reliability (Ranging from 0.767 to 0.917) and intraobserver reliability (0.777 to 0.937), measurement error could not completely be excluded because the joint volume was measured in semi-automated manner. (4) For the evaluation of the degree of capsular contracture, measuring the intra-articular pressure is more accurate method than measuring joint volume. However, it is very difficult to measure intra-articular pressure for every follow-up patients and to measure pressure of specific joint compartment. Therefore, we used joint volume to evaluate the degree of capsular contracture. However, joint volume depends on the expansion behavior of the capsule which is not linear but logarithmic. To overcome this problem, we stopped injection of contrast media at the time patient felt pain and pressure during CTA. Furthermore, we excluded the patient who did not felt pain or pressure and received all 20 mL of contrast media in this study. (5) Clinical scores are not assessed in this study. Study about the influence of joint volume on clinical scores may have induced another clinically meaningful results and further study regarding this issue seems to be useful.
In conclusion, the total joint volume showed positive correlation with postoperative ROM after ARCR. Specifically, among four half compartment joint volumes (anterior, posterior, superior, and inferior), posterior half and inferior half compartment joint volume were related to ROM after ARCR. Considering its frequent incidence and effect on poor clinical outcome, preventing the postoperative stiffness after ARCR is an important issue and perioperative methods to increase the total joint volume, especially the posterior or inferior part of the capsule seems to be useful.