A Comparison of Fixation Methods for Acromioclavicular Joint Separation: A Biomechanical Study

Abstract:

Acromioclavicular (AC) joint dislocations and distal clavicle fractures can be treated in a variety of ways including benign neglect, brace stabilization, or surgical intervention.  Surgical stabilization of the coracoclavicular interval is problematic with no obviously superior method available; however, a percutaneous method would be desirable. 

In a controlled laboratory study we compared the biomechanical characteristics of the traditional Bosworth screw with a suture anchor technique in order to determine if they have comparable strength and stiffness.  The suture anchor technique uses two suture anchors that can be placed percutaneously into the coracoid and the sutures are then tied together over the clavicle.  Results: Our results showed that the Bosworth screw had a mean maximum load at failure of 903(SD=72) N and a stiffness of 200(SD=77) N/mm.  The suture anchor technique had a mean maximum load of 800(SD=182) N and a stiffness of 76(SD=26) N/mm.  The difference in the load at failure was not statistically significantly different but the stiffness of the Bosworth screw was statistically greater, p-value =0.036.  The suture anchor results are not dissimilar to the load to failure of 725 (SD=230) and stiffness of 115.9 (SD =36.2) previously reported by Motemedi upon testing of intact cadaveric coracoclavicular intervals. The double suture anchor technique appears to provide sufficient strength to adequately stabilize the coracoclavicular interval and provides stiffness similar to that of the intact coracoclavicular ligament system. 

We believe that these data suggest a clinical trial may be in order to evaluate the use of the double suture anchor technique for the stabilization of the coracoclavicular interval.

Keywords:

Acromioclavicular joint separation, Bosworth screw, suture anchor, biomechanical testing

J.Orthopaedics 2007;4(4)e8
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Introduction:

Separation of the acromioclavicular (AC) joint can have long-lasting effects and current treatment regimens vary depending on the severity of the injury.  It is accepted by most physicians that type I and II separations should be treated with conservative, non-surgical interventions.  Type III injuries are more controversial with recommendations for both operative or non-operative treatment and some suggesting a regimen of conservative treatment followed by surgical intervention for patients that do poorly with conservative therapy.¹  There is not one surgical method that has been proven to be adequate for all situations, and no research has proven that there is one best method.  In a prospective study, 60 patients with acromioclavicular joint dislocations were randomly distributed for treatment with a broad arm sling or reduction and fixation with a Bosworth screw.  It was shown that conservatively treated patients regained movement significantly more quickly and fully, returned to work earlier and had fewer unsatisfactory results than those having early operation. However, in those with severe dislocations (acromioclavicular displacement of 2 cm or more) early surgery produced better results.¹ This study concluded that conservative management is best for most acute dislocations, but younger patients with severe displacement may benefit from early reduction and stabilization.¹ Surgical repair is also recommended by many physicians for type IV, V, and VI AC joint separations.^5,10  These classifications are based on the extent of injury to the surrounding fascia, ligamentous structures, and the degree and direction of displacement of the clavicle.⁹  There are many different surgical procedures described, which suggests that there is yet to be a definitive surgical method for treating AC dislocations.  Several popular repair and reconstruction techniques include the Weaver-Dunn coracoclavicular ligament transfer, the Bosworth screw, and stabilization using braided polyethylene secured to the clavicle and looped around the base of the coracoid process.⁴ Another stabilization technique uses suture anchors to attach braided polyethylene to the coracoid.^2,11  This technique passes the sutures through drill holes in the clavicle and has been shown to be as stable as suture loops passed under the coracoid. It has the advantage of diminished risk to the musculocutaneous nerve beneath the coracoid. Unlike the Bosworth screw, it does not rigidly fix the AC joint and so does not require removal. The senior author has occasionally and successfully used a similar technique without drill holes in the clavicle, and found that stabilization could even be achieved percutaneously but felt a biomechanical evaluation was in order.

The purpose of this investigation was to evaluate the biomechanical strength of a double coracoid suture anchor method for surgical AC joint stabilization.  This technique can be performed percutaneously by using fluoroscopy to place two suture anchors into the coracoid and then uses #5 Fiberwire™ tied over the clavicle to re-approximate the clavicle and coracoid.  An advantage of this procedure is that it can be performed through a single 5-8mm skin incision which is pulled forward and backward to pass suture anchors anteriorly and posteriorly to the clavicle.  It is anticipated that if such stabilization were performed in the acute phase of AC injury, the AC and coracoclavicular ligaments would likely heal without being sutured and that the cosmetic outcome of a surgical treatment with such a small incision would be desirable in some patients. In addition such stabilization could also be performed during the open or arthroscopic reconstruction of chronic AC separations and might be safer, easier to perform, and more stable than some current techniques that depend on straps passed beneath the coracoid.  In order to determine the feasibility of such treatment, this suture anchor stabilization was biomechanically compared to the strength of the accepted but problematic Bosworth screw technique.

Material and Methods :

Eight synthetic model clavicles and simulated coracoids composed of composite simulated cortical bone (E-glass-filled Epoxy, 3rd generation sawbone 3308, Pacific Research Laboratories, Vashon, Washington) were obtained for this study.  Four coracoids and clavicles were used in group 1, the double suture anchor technique, and four for group 2, the Bosworth screw fixation technique.  All experimental samples were tested with the simulated coracoid and clavicle in correct anatomical position based on averaged measurements of 10 sets of normal shoulder radiographs, and pretensioned to 25 N, evenly distributing the load as described by, Motamedi et al, 2000. Distance from the superior aspect of the coracoid to the inferior aspect of the clavicle was measured after preloading.  The preloaded coracoclavicular distance was set at 10mm in order to simulate normal anatomy as determined from the measurement of the normal shoulder x-rays.  The simulated coracoid bone was constructed by using the medial end of the composite clavicle mounted with its middle, transversely cut surface facing forward.  Its diameter at this point was 11mm and approximated the diameter of the coracoid on our series of x-rays, which averaged 12 mm. It was securely mounted in a 3 in. diameter stainless steel pipe with Bondo (Bondo Corporation, Atlanta, Ga.) leaving 5 cm protruding to simulate the coracoid process.  It was then attached to a material testing machine.  Testing was performed using a MTS Model 812 (Model 812 MTS Systems, Canton, MA).  The synthetic clavicle was attached to the testing frame with a metal hose clamp which compressed it against a hardwood block that had been shaped to fit the clavicles and successfully limited position shifting.  The clavicle frame was connected to the MTS machine by a cast aluminum fixation device.  Using this configuration the coracoid and clavicle were arranged in the correct anatomic position.  Group 1 used a suture anchor technique that requires two suture anchors.  Each suture anchor (3.5mm, Corkscrew^TM, Arthex, Naples, Florida) was threaded with #5 Fiberwire (Arthex Inc, Naples, Florida) suture.  One suture anchor was placed into the coracoid anterior to the clavicle and a second anchor posterior to the clavicle, the distance between the two suture anchors was 25 mm.  The suture from the posterior anchor was tied to the suture from the anterior anchor over the top of the clavicle with 6 standard surgical knots.  Group 2 contained four clavicles and simulated coracoids which were stabilized using the Bosworth screw technique.  We were unable to obtain a commercially made “Bosworth screw” and so a 4.5mm cortical screw was placed through the clavicle into a predrilled 3.2mm drill hole in the center of the coracoid similar to the technique described by Bosworth.³ The clavicle and coracoid were again fixed 10mm apart.  Each coracoid and clavicle was then tested to failure with force applied at a rate of 4mm/min in a superior direction.  The force (N) at failure and stiffness (N/mm) were recorded for each sample.  Load at failure was obtained directly from the MTS machine, stiffness was calculated from the slope of the linear portion of the load-displacement curve which was obtained via linear regression.⁸

            Data was compiled using Microsoft Excel software (Microsoft Corp, Redmond, Washington).  The mean values with standard deviations were calculated for each group.  T-tests were then done to statistically evaluate differences between the groups. These data were also compared to data from Motemedi regarding the normal strength and stiffness of the uninjured coracoclavicular complex.

Results :

Load at failure (N) and stiffness (N/mm) for each group are summarized in Table 1.  Each value is reported as the mean of 4 specimens +/- the standard deviation.  The double suture anchor technique had a strength of 800.08 +/-182.14N versus 903.21 +/-72.34N for the Bosworth screw (p=0.13).  The stiffness of the suture anchor technique was 76.38 +/-26.03N/mm and was significantly smaller than the stiffness of the Bosworth screw at 200.20 +/- 77.18N/mm (p =.036). 

The modes of failure from the Bosworth screw technique were two coracoid fractures, one screw pull out failure, and one run stopped when the safety load limit of 1000N was reached.  The double suture anchor technique failed three times due to fracture of the coracoid and one time due to clavicle fracture. Motemedi et al. reported the modes of failure for the intact coracoclavicular ligament complex due to mid-substance tear eight times and fracture of the cadaver coracoid one time.⁸

Discussion:

Although some surgeons posit that AC injuries cannot heal and therefore must be reconstructed, a possible goal in an acute injury may be to stabilize the AC joint and coracoclavicular interval for a period of time in hopes that the ligamentous injury can heal. The double suture anchor technique is attractive in that it allows a cosmetically desirable percutaneous stabilization of the coracoclavicular interval but does not produce rigid fixation and the need for removal of as with the Bosworth screw device. The goal in augmenting a coracoclavicular ligament reconstruction is to protect the reconstruction during its healing phase, when it is weakest and further damage could occur.⁷ Ideally any stabilization technique should recreate the biomechanical characteristics including load to failure and stiffness of the intact coracoclavicular ligament complex in a normal uninjured person.⁵ Our study was aimed at evaluating and comparing the strength, stiffness, displacement, and mode of failure of the double suture anchor technique.  The Bosworth screw was used as a “gold standard” as it has been evaluated in several studies but the biomechanical characteristics of suture anchor stabilization of the coracoclavicular interval has not been compared to it.^6,8  We found that this particular double suture anchor technique was in the same range for strength but significantly less stiff than the Bosworth screw technique. When our values are compared to the biomechanical data of the intact acromioclavicular joint reported by Motemedi et al. the values for the failure load of the double suture anchor technique at 800.085 +/-182.143N compare favorably to the intact coracoclavicular complex data at 724.9 +/-230.9N.  The stiffness of the suture anchor technique at 76.387 +/-26.034N/mm was lower than, but in the range of the intact coracoclavicular complex’s stiffness of 115.9 +/- 36.2N/mm. Our experiment is unique as compared to other studies in our use of composite bone.  The current literature only reports studies using cadaveric bone for testing, which is problematic because of cadaveric bone weakness due to age, prior injury, or decreased density secondary to osteoporotic changes.  The majority of studies have used cadaveric bone from donors ranging in age from 50 years old to 80 years old.  This does not accurately approximate the bone strength or screw holding power of the typical young, athletic patient who sustains an AC joint separation. 

The placement of the suture anchor into the superior aspect of the coracoid puts the musculocutaneous nerve beneath the coracoid at diminished risk as compared to looping a suture underneath the coracoid.  There is only one procedure needed to place the suture anchor into the coracoid, the sutures can then be left in the coracoid indefinitely, unlike the Bosworth screw technique where the screw must be surgically removed in order to decrease the chance of bone fatigue or bone fracture.  In addition the Bosworth screw technique markedly limits joint motion and may be so stiff that the lack of motion impedes the ligament’s healing or the maturation of the tissues of a reconstruction.   

            The mean load of failure we obtained for the Bosworth screw technique were much higher than those obtained by Motemedi et al., 903N vs. 390N.⁸  The mode of failure most common during our testing was coracoid fracture while the main mode of failure in Motemedi’s trial was screw pull out.⁸  These differences are most likely due to the inherent strength of the composite bone vs. the weakness of the cadaveric bone.  The stiffness of the Bosworth screw technique is much larger than both the intact coracoclavicular complex and the suture anchor techniques.  This may create problems with maintaining normal function and range of motion.  Abducting the shoulder to a full 180 degrees requires a 20 degree rotation at the AC joint.  The Bosworth screw technique could be too stiff and could inhibit this movement creating functional problems for the patient.

             Motemedi et al. describes a technique of drilling a hole through the coracoid and placing a braided polyethylene through the drill hole.  Their results returned a mean failure load of 986.1 N and a stiffness of 99.8 N.  Drilling through the entire coracoid places the musculocutaneous nerve inferior to the coracoid at risk, which makes this a riskier procedure.  It may also be a more technically demanding surgery as the coracoid is put at risk of fracture when the hole is placed through it.    

            The double anchor technique used two suture anchors placed into the coracoid and has many positive aspects as a potential method for AC joint repair or reconstruction.  It can be carried out through a small 5mm incision, minimizes the jeopardy to surrounding neurologic, vascular, or musculoskeletal structures and does not require removal.  The strength and stiffness is similar to that of the intact coracoclavicular ligament complex.  Clinical studies are needed to evaluate how the decreased stiffness will affect the clinical outcomes of the double suture anchor technique. 

Table 1. Data from the 2 experimental groups including Failure load (N), Stiffness (N/mm), and mode of failure.

Reference :

1. Bannister GC, Wallace WA, Stableforth PG, Hutson MA. The management of acute acromioclavicular dislocation. A randomized prospective controlled trial. J Bone Joint Surg. 1989 Nov. 71(5):848-50. PMID: 2684990

2. Breslow MJ, Jazrawi LM, Bernstein AD, Kumm FJ, Rokito AS. Treatment of acromioclavicular joint separations: Suture or suture anchor? J Shoulder Elbow Surg. 2002 May-Jun;11(3):225-9. PMID:12070493

3. Bosworth, BM. Acromiclavicular Separation: New method of repair. Surg Gynecol  Obstet. 1941. 73: 866-871 

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9. Richards RR. Acromioclavicular joint injuries. Instr Course Lect. 1993;42:259-69. PMID: 8463674 

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This is a peer reviewed paper 

Please cite as : Hromadka  Michael : A Comparison of Fixation Methods for Acromioclavicular Joint Separation: A Biomechanical Study

J.Orthopaedics 2007;4(4)e8

URL: http://www.jortho.org/2007/4/4/e8

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