Orthopedics

Feature Article 

Biomechanical Evaluation of Unicortical Stress Risers of the Proximal Humerus Associated With Pectoralis Major Repair

David J. Wilson, MD; Brian P. Milam, MD; William F. Scully, MD; Todd P. Balog, MD; Kyong S. Min, MD; Christopher S. Chen, MD; Bryant G. Marchant, MD; Edward D. Arrington, MD

Abstract

Proximal humerus fracture after pectoralis major tendon repair has been recently reported. Although this complication is rare, it may be possible to decrease such risk using newer techniques for myotenodesis. This study was designed to evaluate various unicortical stress risers created at the proximal humeral metadiaphysis during myotenodesis for repair of pectoralis major ruptures. A simulated pectoralis major myotenodesis was performed using fourth-generation Sawbones (N=30). Using previously described anatomic landmarks for the tendinous insertion, 3 repair techniques were compared: bone trough, tenodesis screws, and suture anchors (N=10 each). Combined compression and torsional load was sequentially increased until failure. Linear and rotational displacement data were collected. The average number of cycles before reaching terminal failure was 383 for the bone trough group, 658 for the tenodesis group, and 832 for the suture anchor group. Both the tenodesis and the suture anchor groups were significantly more resistant to fracture than the bone trough group (P<.001). The suture anchor group was significantly more resistant to fracture than the tenodesis group (P<.001). All test constructs failed in rotational stability, producing spiral fractures, which incorporated the unicortical defects in all cases. When tested under physiologic parameters of axial compression and torsion, failure occurred from rotational force, producing spiral fractures, which incorporated the unicortical stress risers in all cases. The intramedullary suture anchor configuration proved to be the most stable construct under combined axial and torsional loading. Using a bone trough technique for proximal humerus myotenodesis may increase postoperative fracture risk. [Orthopedics. 2017; 40(5):e801–e805.]

Abstract

Proximal humerus fracture after pectoralis major tendon repair has been recently reported. Although this complication is rare, it may be possible to decrease such risk using newer techniques for myotenodesis. This study was designed to evaluate various unicortical stress risers created at the proximal humeral metadiaphysis during myotenodesis for repair of pectoralis major ruptures. A simulated pectoralis major myotenodesis was performed using fourth-generation Sawbones (N=30). Using previously described anatomic landmarks for the tendinous insertion, 3 repair techniques were compared: bone trough, tenodesis screws, and suture anchors (N=10 each). Combined compression and torsional load was sequentially increased until failure. Linear and rotational displacement data were collected. The average number of cycles before reaching terminal failure was 383 for the bone trough group, 658 for the tenodesis group, and 832 for the suture anchor group. Both the tenodesis and the suture anchor groups were significantly more resistant to fracture than the bone trough group (P<.001). The suture anchor group was significantly more resistant to fracture than the tenodesis group (P<.001). All test constructs failed in rotational stability, producing spiral fractures, which incorporated the unicortical defects in all cases. When tested under physiologic parameters of axial compression and torsion, failure occurred from rotational force, producing spiral fractures, which incorporated the unicortical stress risers in all cases. The intramedullary suture anchor configuration proved to be the most stable construct under combined axial and torsional loading. Using a bone trough technique for proximal humerus myotenodesis may increase postoperative fracture risk. [Orthopedics. 2017; 40(5):e801–e805.]

Although historically considered a rare injury, pectoralis major muscle ruptures continue to be reported with increasing frequency in the orthopedic and sports literature.1–7 The injury typically results from eccentric loading of an externally rotated, extended, and abducted arm.4,7 Although a variety of injury mechanisms have previously been described, bench-pressing is currently responsible for most of these injuries.4,8 An increased incidence of pectoralis major injury during participation in both professional and amateur sports, such as football, basketball, and rodeo steer wrestling, has also been noted in recent years.4,9–12 Most of those sustaining this injury are young, muscular individuals, but pectoralis major ruptures have also been reported in the elderly population.7,13–15 Rupture at the laminated insertional footprint on the proximal humerus is the most common injury location, but tears at the sternal origin, myotendinous junction, and midsubstance of the muscle also occur.4,7,8,16

The pectoralis major muscle has a broad origin across the chest with 2 distinct heads, identified as the clavicular head superiorly and the sternocostal head inferomedially.4,7 The muscle spans across the superolateral anterior chest wall and inserts on the proximal humerus, just lateral to the bicipital groove4,7 (Figure 1).

Illustration of the laminated tendinous insertion of the pectoralis major.

Figure 1:

Illustration of the laminated tendinous insertion of the pectoralis major.

In an anatomical cadaveric study, Carey and Owens17 further defined the long and narrow insertion of the pectoralis major on the proximal humerus, reporting that the footprint averages 72.3±12.3 mm in length and 1.4±0.2 mm in width. This convergence of the tendinous fibers at the muscle insertion has been described as 3 distinct laminae.4,18 The inferior division of the sternocostal head coalesces with the external abdominal oblique aponeurosis to form the posterior lamina. The superior division (manubrial) of the sternocostal head inserts as the middle lamina. The insertion of the clavicular head forms the superficial lamina.4,18

Nonoperative management of pectoralis major ruptures is primarily reserved for elderly and/or low-demand individuals.4,7 Some individuals with variations on the typical injury pattern, such as partial ruptures or midsubstance muscle belly tears, may also be preferentially treated without surgery.4,7 Although some chronic tears are treated nonoperatively because of the less predictable and less than satisfactory results associated with repair of this subset of injuries, many patients with chronic tears have been shown to benefit from surgical treatment.4,19,20 In young, active patients presenting acutely with a pectoralis major insertional rupture, surgical treatment is indicated, given the significant improvements in patient satisfaction, strength, cosmetic appearance, and return to work/sport shown with operative treatment compared with nonoperative management.2,4–8,18–21

In the repair of acute insertional pectoralis major ruptures, the goals of surgical treatment include anatomic reproduction of the tendinous insertion and restoration of adequate strength at the repair site.4,7 The latter aim is particularly important regarding safely initiating early motion and strengthening postoperatively.22 Several methods for surgical repair of these injuries have been described to meet these objectives. Historically, treatment involved suture-only repair through transosseous drill holes.3,23 This technique was later modified, by Wolfe et al18 and Schepsis et al,21 to incorporate a bone trough. In the Schepsis et al21 technique, the ruptured tendon is secured with high-tensile strength, braided, nonabsorbable sutures and fed into a 3- to 5-cm × 5-mm trough by passing the suture strands through unicortical drill holes that exit lateral to the trough. The sutures are then tied over a “boney bridge.” Other repair techniques involving screws and washers and barbed staples have been reported.24,25 With the advent of new tendon fixation implants for various sports and trauma procedures, other successful repair techniques incorporating suture anchors and cortical buttons have been described.19,26–29 A recent retrospective review by Nute et al8 found that the most commonly used fixation method for patients treated in military hospitals between 2008 and 2013 used suture anchors, with the second most common fixation using a bone tunnel technique.

One surgical feature common among all of these techniques is the creation of one or multiple cortical defects of varying size as part of the repair. Recently, a case of proximal humerus fracture following pectoralis major myotenodesis, using the aforementioned bone trough technique, was described.30 Two additional case reports of proximal humerus fracture following subpectoral keyhole biceps tenodesis have been published.31,32 Although the incidence of this complication is low, proximal humerus fracture following these myotenodesis procedures results in significant morbidity and delays in recovery.30–32 With multiple surgical procedures available for pectoralis major repair, the current authors hypothesized that those techniques requiring larger bone defects result in increased stress risers across the proximal humerus and increased risk of fracture. Additionally, the authors sought to explore whether unicortical defects that are “filled” by implants that reside flush with the surrounding cortex, such as those created by large biocomposite anchors or tenodesis screws, are more resistant to fracture than repairs that create “unfilled” defects, such as intramedullary-residing suture anchors and cortical buttons, despite the larger drill holes often required for the former group compared with the latter.27–29 No previous biomechanical data exist in relation to these specific clinical issues.

Materials and Methods

A simulated pectoralis major myotenodesis was performed using 30 fourth-generation Sawbones (Pacific Research Laboratories, Inc, Vashon, Washington). Current fourth-generation Sawbones closely match the modulus of elasticity, and as a result the torsional and compressive stiffness, of living human cortical and cancellous bone. This Sawbone model has been successfully used to evaluate component stability of shoulder arthroplasty implants.33

Using previously described landmarks for the tendinous insertion, 3 different repair techniques were compared to analyze the type (filled vs unfilled) and size of unicortical defects created during these procedures.17 In the first group (N=10), a 5×40-mm bone trough was created using an oscillating saw with four 2.0-mm unicortical drill holes oriented longitudinally and exiting 1 cm lateral to the trough, as described by Schepsis et al.21 The second group (N=10), or filled grouping, consisted of two 7×15-mm biocomposite tenodesis screws (MILAGRO Bioreplaceable Interference Screws; DePuy Mitek, Warsaw, Indiana) inserted into 8-mm drill holes. The third group (N=10) used three 2.8-mm suture anchors (Lupine Loop Anchor with Orthocord; DePuy Mitek) designed to dock within the medullary space, leaving the 2.9-mm insertion drill holes unfilled. Each proximal humerus construct was mounted in plaster and tested using a dynamic servohydraulic loading device (8521 Servohydraulic Dynamic Load Device; Instron, Norwood, Massachusetts) (Figure 2).

Illustration depicting the position and pattern of the bone trough (A), tenodesis screws (B), and suture anchors (C) along the insertional footprint of the pectoralis major tendon, as described by Carey and Owens.17

Figure 2:

Illustration depicting the position and pattern of the bone trough (A), tenodesis screws (B), and suture anchors (C) along the insertional footprint of the pectoralis major tendon, as described by Carey and Owens.17

Loads were initiated at a low physiologic threshold and modeled to simulate using one's arms to rise from a chair. Combined compression and torsion were delivered with each humerus mounted in a 20° abducted position. The compressive starting load was set at 50 N and the torsional starting load at 5 Newton meters (Nm). Increases of 50 N of compressive and 5 Nm of torsional load were applied every 100 cycles. Loads were sequentially increased until failure. Linear and rotational displacement data were collected for each cycle until failure.

At a power of 0.8 and an alpha error of 5%, a sample size of 10 Sawbones was sufficient to determine a statistically significant difference between groups. Standard deviation was calculated. Confidence intervals (95%) were calculated assuming normal distribution. The Student's t test was used for statistical comparison, and a z test was used to detect outliers. P<.05 was considered significant.

Results

The average number of cycles that the proximal humerus models withstood before reaching terminal failure was 383 (SD, 48) at a combined average of 200 N of compressive and 20 Nm of torsional load applied for the bone trough group (group I). The tenodesis group (group II, filled) failed at an average of 658 cycles (SD, 79), with a combined average of 350 N of compressive and 35 Nm of torsional load applied. The suture anchor group (group III, unfilled) failed at an average of 832 cycles (SD, 94), with a combined average of 450 N of compressive and 45 Nm of torsional load applied. Catastrophic failure was observed in all samples, and thus load-to-failure data were obtained for each sample.

Both the tenodesis and the suture anchor groups were significantly more resistant to fracture than the bone trough group (P<.001). The suture anchor or unfilled group was significantly more resistant to fracture than the filled tenodesis group (P<.001). There were no outliers in the data set (Figure 3).

Load-to-failure diagram with 95% confidence intervals, defined by number of cycles required to produce fracture, for the 3 groups tested. Abbreviations: BT, bone trough; SA, suture anchors; TS, tenodesis screws.

Figure 3:

Load-to-failure diagram with 95% confidence intervals, defined by number of cycles required to produce fracture, for the 3 groups tested. Abbreviations: BT, bone trough; SA, suture anchors; TS, tenodesis screws.

The timing of failure in all 30 models was sudden, progressing over just a few cycles. No gradual progression to failure was seen. All test constructs failed in rotational stability, producing spiral fractures, which incorporated the various unicortical defects in all cases (Figure 4).

Photograph of failed testing sample, showing the spiral fracture pattern incorporating the bone trough, identical to that reported clinically by Silverstein et al.30

Figure 4:

Photograph of failed testing sample, showing the spiral fracture pattern incorporating the bone trough, identical to that reported clinically by Silverstein et al.30

Discussion

With the increasing frequency of pectoralis major muscle ruptures and the multitude of available surgical treatment options and implants, biomechanical testing has recently been initiated to compare the strength of repair techniques. Hart et al34 and Sherman et al35 did not find statistically significant differences in load-to-failure testing between the trough technique and either large defect-filling suture anchors or intramedullary cortical button constructs (ie, unfilled). Conversely, Rabuck et al36 did find the trough technique to be statistically stronger compared with endosteal cortical buttons and suture anchor constructs, but all 3 techniques remained significantly weaker than the intact tendon. The mode of failure in most of the repairs in the 3 cadaveric studies was suture pulling through the tendon.34–36 However, Rabuck et al36 reported 1 proximal humerus fracture in the bone trough group.

Biomechanical testing to specifically evaluate the risk of fracture associated with these pectoralis major repair techniques has not been previously reported. The current study sought to evaluate the role of proximal humerus myotenodesis sites as unicortical stress risers under low physiologic stress and advanced stress to failure. When tested under physiologic parameters of axial compression and torsion, failure occurred from rotational force, producing spiral fractures, which incorporated the unicortical stress risers in all cases.

Conclusion

The bone trough group, which had the largest cumulative unicortical defect with the 5×40-mm trough and four 2.0-mm drill holes, failed statistically (P<.001) earlier in the testing series and under significantly (P<.001) lower compressive and rotational force than the other groups. Comparing the other 2 test groups, the suture anchor group (group III), consisting of three 2.9-mm defects, proved to be more resistant (P<.001) under combined axial and torsional loading than the tenodesis group (group II) with its two 8-mm unicortical defects. Intuitively, smaller unicortical defects produced more fracture-resistant constructs when tested under these conditions.

Despite filling the defect and thereby theoretically diminishing the stress riser created by the drill holes, the tenodesis group (group II), with much larger drill holes, proved to be less fracture resistant (P<.001) than the suture anchor group. These data suggest that, in reducing fracture risk, the potential biomechanical benefit of filling the defect is less important than minimizing the overall defect size.

The limitations of this study included the use of a fourth-generation Sawbone model and the smaller samples. Additionally, given recent research into the use of suture anchor constructs, an additional suture anchor test group may have yielded valuable information pertinent to modern repair techniques.8

Multiple options exist for pectoralis major insertional repair. Although the strongest technique has not been clearly elucidated, the results of this study favor using smaller implants and minimizing the size of cortical defects. Using a bone trough technique for pectoralis major repair may increase postoperative fracture risk.

References

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Authors

The authors are from Bassett Army Community Hospital (DJW), Wainwright, Alaska; the Department of Orthopedics (BPM, CSC, BGM, EDA), Madigan Army Medical Center, Tacoma, Washington; Martin Army Community Hospital (WFS), Fort Benning, Georgia; Blanchfield Army Community Hospital (TPB), Fort Campbell, Kentucky; and Brian Allgood Army Community Hospital (KSM), Seoul, Korea.

Drs Balog and Marchant are previous Blue Ribbon Article Award recipients (Orthopedics, March/April 2017).

The authors have no relevant financial relationships to disclose.

The views expressed in this manuscript are those of the authors and do not reflect the official policy of the Department of the Army, Department of Defense, or US Government. All authors are employees of the US Government. This work was prepared as part of their official duties, and as such, there is no copyright to be transferred.

This research was made possible through material and facility support from the Andersen Simulation Center, Madigan Army Medical Center, Tacoma, Washington. This research project was funded through an Orthopedic Research and Education Foundation (OREF) 2012–2013 Resident Research Grant. All implants used in this study were obtained through a transfer of use agreement with Mitek (DePuy Mitek, Warsaw, Indiana) managed through the Madigan Army Medical Center, Department of Clinical Investigation, Tacoma, Washington.

Correspondence should be addressed to: Brian P. Milam, MD, Department of Orthopedics, Madigan Army Medical Center, 9040 Jackson Ave, Tacoma, WA 98431 ( Brian.p.milam2.mil@mail.mil).

Received: December 16, 2016
Accepted: July 05, 2017
Posted Online: August 18, 2017

10.3928/01477447-20170810-02

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