The acromioclavicular (AC) joint is formed by the

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1 9(2):80 84, 2008 T E C H N I Q U E The Distal Clavicle Morphology Xiao L. Wu, MBBS and George A. C. Murrell, MD, DPhil Orthopaedic Research Institute St George Hospital Campus University of New South Wales Sydney, Australia Ó 2008 Lippincott Williams & Wilkins, Philadelphia ABSTRACT Background: To our knowledge, no previous studies have been conducted to comprehensively examine the morphology of the distal clavicle. The purpose of this study was to examine the distal clavicle morphology by assessing the dimensions and the angles of the distal clavicle in relation to the design of a potential distal clavicle prosthesis. Methods: Twenty cadaver clavicles were examined. The dimensions of the distal clavicular joint facet were directly assessed using a digital micrometer. Other angles and dimensions of the distal clavicle were assessed by taking measurements from the photographs of these clavicles taken from the anterior, the superior, and the inferior views. Radiographs were used to assess the cortices of the distal clavicle. Results: The average dimension of the distal clavicle joint facet was 14.7 mm (95% confidence interval [CI], 13.2Y16.1 mm) by 10.0 mm (95% CI, 9.2Y10.9 mm). The average dimension of the clavicle at 2 cm from its distal joint facet was 19.5 mm (95% CI, 17.6Y21.4 mm) by 10.0 mm (95% CI, 9.0Y11.0 mm). The average distal curvature length was 38.7 mm (95% CI, 36.7Y40.6 mm). The average distal curvature angle was 81 degrees (95% CI, 75Y87 degrees). The average normal length was 34.2 mm (95% CI, 30.8Y37.5 mm). The lateral tilting angle was measured to have an average value of 81 degrees (95% CI, 78Y84 degrees). The cortices of the distal clavicle were deficient. Conclusions: The results of this study suggest that the dimension of the distal clavicle joint facet, 13.2 to 16.1 mm (95% CI) by 9.2 to 10.9 mm (95% CI), would provide reference values for the size of the head piece of a distal clavicle prosthesis. The distal curvature length of 36.7 to 40.6 mm (95% CI) and the normal length of 30.8 to 37.5 mm (95% CI) would provide reference values for the length of the stem part of the prosthesis. The lateral tilting angle 78 to 84 degrees (95% CI) should be met to reproduce the natural distal clavicle anatomy. Reprints: George A. C. Murrell, MD, DPhil, Department of Orthopaedic Surgery, St George Hospital Campus, University of New South Wales, Kogarah, Sydney 2217, New South Wales, Australia ( murrell.g@ori.org.au). Keywords: shoulder, clavicle, prostheses, acromionclavicular joint, anatomy The acromioclavicular (AC) joint is formed by the distal clavicle articulating with the medial facet of the acromion. Disorders of this joint are relatively common and can cause significant disability. 1 Disorders of the AC joint include instability (dislocations) and degenerative changesvoveruse or posttraumatic osteoarthritis and osteolysis of the joint. 2,3 The AC joint is also predisposed to conditions that affect other joints in the body, including infection, inflammatory arthritis, and crystal deposits. 3 Most AC joint pathology can be managed nonoperatively. A surgical procedure for degenerative conditions of the AC joint involves a resection of 1 to 2 cm of the distal clavicle. 3 This can be performed by open procedure (Mumford procedure) or by arthroscopy. The resection has been reported to relieve pain and to improve function. 4,5 The most common complication postoperatively is residual pain of the AC joint. Residual pain may be due to inadequate resection of the bone such that there is still bone-to-bone contact at the AC joint. 3,6 The opposite, resection of too much of the distal clavicle, often disrupts the integrity of the AC joint ligaments and results in instability. Instability is more common in patients who have had AC joint separation or ligament disruption in the past. 3,7Y9 One potential way of managing these problems or AC joint pain that has failed nonoperative management may be a distal clavicle prosthesis. However, it should be emphasized here that there are no data to suggest that a distal clavicle prosthesis may successfully solve the problems. To design a distal clavicle prosthesis, it is important to understand the morphology of the distal clavicle, and this is the focus of this article. METHODS Twenty-nine clavicles from cadavers were identified, of which 20 were preserved well enough to allow measurements to be taken on the distal portion of the clavicle. The 9 clavicles excluded from this study had their distal portion slightly damaged such that accurate measurements could not be taken. Of the 20 clavicles 80

2 The Distal Clavicle Morphology FIGURE 1. A, Superior view of the clavicle. B, Inferior view of the clavicle. C, Anterior view of the clavicle. included in this study, there were 10 left and 10 right clavicles. All bones were defleshed and decapsuled, and the cartilaginous articular surface removed. Assessing the Distal Clavicle Joint Facet The dimensions of the distal (AC) joint facet of the clavicles were measured directly with a digital micrometer (Mitutoyo CD-6µPS, Mitutoyo Co, Tokyo, Japan). The dimension that horizontally bisected the distal facet was measured as the length. The dimension that vertically bisected the distal facet was the width. Two diagonal measurements were also taken; these diagonals were taken as lines that were inclined at 45 degrees anteriorly and posteriorly to the width. All measurements were performed on 2 separate occasions, and the average values were finally recorded. Assessing the Lengths and Angles of the Distal Clavicle To assist other measurements, we took photographs of the clavicles using a digital camera (Nikon MH-18a; Nikon Corp, Tokyo, Japan). These photographs included superior, inferior, and anterior views. The focal distance was chosen to be 1.2 m so that the view size of the camera FIGURE 2. AYC, Angles and lengths of the distal clavicle. Volume 9, Issue 2 81

3 Wu et al FIGURE 3. A, An illustrative radiograph of the clavicle, superior view. B, An illustrative radiograph of the clavicle, lateral view. was approximately cm. The background that the clavicles were placed on (an A4-size paper) occupied the entire view of the field, and the clavicles were placed in the center of the field. A ruler was included in the photos as a scale (Figs. 1AYC). The following angles and lengths were then measured from the photographs: from the anterior view, the angle (E) between the distal facet of the clavicle and its horizontal bisector; the superior-inferior transsectional thickness at the position 2 cm from the distal facet (t); from the superior and the inferior views, the diagonal length from the midpoint of the distal facet to the vertex of the distal clavicle curvature (d); the angle of this diagonal length (8) as well as the anterior-posterior width at 2 cm from the distal facet (w); lastly, from the superior and the inferior views, a perpendicular line was drawn from the midpoint of the distal facet to the posterior border of the clavicle, and the length of this line was recorded (p). Schematic drawings are shown in Figures 2AYC. All measurements were performed on 2 separate occasions, and the average values were recorded. Assessing the Cortices of the Distal Clavicle Radiographs were taken to assess the cortical bones of these clavicles using an FH-21, P-20 radiograph machine (Shimadzu Corp, Kyoto, Japan). The settings were adjusted until the cortical bone and the medulla can be adequately distinguished (60 kv, 16 ma; Figs. 3AYB). Both the superior and the anterior views were imaged. Thickness of the bone cortex was assessed at 2 positions, at the vertex of the distal curvature and at the midpoint of the clavicle shaft. The anterior and the posterior cortical thicknesses were measured from the superior view. The superior and the inferior cortical thicknesses were FIGURE 4. Distal clavicle morphologies. assessed from the anterior view. The distance from the distal facet to the start of the cortical bone was also measured in the superior and the anterior views. Statistical Analysis The mean, the range, the 95% confidence interval (CI), and the 99% CI were calculated for all measurements. Variations were assessed for significance using 2-tailed, unpaired Student t tests. RESULTS All clavicles were observed to have double curvatures, with the medial one curved anteriorly and the lateral one curved posteriorly. This finding was consistent with the previous literature. 10,11 The Distal Clavicle Facet The distal clavicle from the superior view had a triangular shape. The anterior border of this triangle corresponded to the superior border of the AC joint facet; hence, the joint facet of the distal clavicle was orientated anteriorly. The morphology of the joint facet showed variations. The morphology was classified into 2 basic shapesvoval and polygonal (Fig. 4). Of the 20 clavicles examined, 11 (55%) were classified as oval, and 9 (45%) were classified as polygonal. In addition, 2 (18%) of 11 clavicles classified as oval and 2 (22%) of 9 clavicles classified as polygonal were noted to have the anterior dimensions slightly smaller than the posterior ones. The measurements of the distal clavicle facet are shown in Table 1. The distal dimensions of the 20 clavicles were then analyzed separately based on the morphology of the TABLE 1. Distal Clavicle Facet Measurements Length, mm 9.5Y Y Y17.0 Width, mm 7.1Y Y Y11.5 Anterior-posterior 9.4Y Y Y14.5 diagonal, mm Posterior-anterior diagonal, mm 9.4Y Y Y

4 The Distal Clavicle Morphology TABLE 2. Oval Distal Morphology Versus Polygonal Distal Morphology Type 1, flat oval Length, mm 9.5Y Y Y16.9 Width, mm 7.1Y Y Y10.2 Type 2, polygonal Length, mm 10.3Y Y Y19.4 Width, mm 8.7Y Y Y13.4 distal facet. The length-to-width ratio of each type was calculated. The mean length-to-width ratio of the oval type was greater than the polygonal type by 0.3 (P = 0.015). These are shown in Tables 2 and 3. The Lengths and Angles of the Distal Clavicle The range, the mean, the 95% CI, and the 99% CI of the lateral tilting angle (E) of the distal clavicle, the thickness (t), the diagonal lengthvdistal curvature length (d) and the angle of this diagonal lengthvdistal curvature angle (8), the width (w), and the normal length (p) of the distal clavicle facet are outlined in Table 4. Cortices of the Distal Clavicle Radiographs of the clavicles indicated the distal clavicles were not covered by dense cortices. The cortex of the distal clavicle started to become visible at an average distance of 7.8 mm (95% CI, 5.8Y9.9 mm) from the distal facet superiorly and inferiorly and showed more deficit anteriorly and posteriorly, starting at a mean distance of 12.3 mm (95% CI, 10.4Y14.1 mm) from the distal facet. The cortex at the distal clavicle was very thin but reached the full thickness quite quickly before the distal curvature and resumed a uniform thickness until approaching the sternal end. The variation in the cortical bone thickness from bone to bone was not great, and the values are summarized in Table 5. DISCUSSION Overall, the variations in the morphology of the 20 clavicles examined were reasonably large. To our knowledge, this is the first assessment of the distal clavicle morphology. The mean, range, 95% CI, and 99% CI for the facet dimensions including the length, the width, and the 2 diagonal lengths may provide reference values for the size of the head piece of a potential distal clavicle prosthesis. TABLE 3. Length-to-Width Ratio for the 2 Types of Morphology Mean Difference in Mean P Type 1, oval Type 2, polygonal 1.3 TABLE 4. Angles and Lengths of the Distal Clavicle Lateral tilting angle 65Y Y84 76Y86 (E), degrees Thickness (t), mm 6.1Y Y Y11.7 length (d), mm 32.2Y Y Y42.0 angle (8), degrees 47Y Y87 71Y92 Width (w), mm 13.9Y Y Y22.7 Normal length (p), mm 18.3Y Y Y39.8 Two types of morphology of the distal clavicle facet were observed: a polygonal one having a longer width relative to the length compared with an oval one. These 2 morphologies may need to be taken into consideration for the design of a prosthesis. Two measurements taken in the study provide reference values for the length of the shaft component of a potential prosthesis. One is the distal curvature length (d), and the other is the normal length (p). The distal curvature length (mean, 38.6 mm) was longer than the normal length (mean, 34.2 mm); however, the former made a variable (usually acute) angle with the articular surface and may be very close to the anterior border of the clavicle. Although the normal length was slightly shorter, it was perpendicular to the distal facet and approximately bisected the distal clavicle. Nevertheless, it is important to note whichever length is to be adopted for designing the shaft of the prosthesis; the thickness of the posterior cortex must be taken into account as the posterior cortex (2.3Y3 mm) was included in these measurements. The width (w) of the distal clavicle (at 2 cm) and the thickness (t) of the distal clavicle (at 2 cm) together give reference to the cross-sectional area of the clavicle 2 cm from its acromial joint facet: 19.5 mm (95% CI, 17.6Y21.4 mm) by 10.0 mm (95% CI, 9.0Y11.0 mm). If the thickness of the cortex anteriorly, posteriorly, superiorly, and inferiorly is taken into account, then the cross-sectional area of the medulla of the clavicle at this point can be estimated. TABLE 5. Cortical Bone Thickness Range Mean 99% CI Superior, mm 0.5Y Y1.8 Inferior, mm 0.5Y Y1.8 Anterior, mm 1.0Y Y2.7 Posterior, mm 0.5Y Y2.3 Midshaft Superior, mm 1.0Y Y2.4 Inferior, mm 1.0Y Y2.3 Anterior, mm 1.0Y Y2.5 Posterior, mm 1.5Y Y2.5 Volume 9, Issue 2 83

5 Wu et al The AC joint line is inclined, and this is reflected by the lateral tilting angle measured in our study. The variation of the lateral tilting angle (E) was small, with a 95% CI of 78 to 84 degrees. Berkowitz et al 13 reported similar results with regard to this angle. The lateral titling angle may need to be considered in the design of the prosthesis to reproduce the joint orientation and integrity. The distal clavicle was found to lack a thick cortex; that is, the cortex started to become visible at a mean distance of 12.3 mm anteriorly-posteriorly and at a mean distance 7.8 mm superiorly-inferiorly from the distal end. This may have implications on how well the distal clavicle can hold the prosthesis. In summary, the variations in the measurements of the distal clavicle morphology, including the distal clavicle facet dimensions, the distal clavicle dimensions at 2 cm from the distal facet, the lateral tilting angle, and the distal curvature length and angle, as well as the normal length, were reasonably large. These measurements of angles and lengths may provide reference values for the design of a potential distal clavicle prosthesis. ACKNOWLEDGMENTS The authors thank the staff of the Anatomy Department of the University of New South Wales for providing access to specimens. REFERENCES 1. Montellese P, Dancy T. The acromioclavicular joint. Prim Care. 2004;31(4):857Y Cahill BR. Osteolysis of the distal part of the clavicle in male athletes. J Bone Joint Surg Am. 1982;64:1053Y Nuber GW, Bowen MK. Arthroscopic treatment of acromioclavicular joint injuries and results. Clin Sports Med. 2003; 22(2):301Y Flatow EL, Cordasco FA, Bigliani LV. Arthroscopic resection of the acromioclavicular joint. Am J Sports Med. 1993;21:71Y Snyder SJ, Banas MP, Karzel RP. The arthroscopic Mumford procedure: an analysis of results. Arthroscopy. 1995;11:157Y Shaffer B. Painful condition of the acromioclavicular joint. J Am Acad Orthop Surg. 1999;7(3):176Y Renfree KJ, Wright TW. Anatomy and biomechanics of the acromioclavicular and sternoclavicular joints. Clin Sports Med. 2003;22(2):219Y Cook FF, Tibone JE. The Mumford procedure in athletes: an objective analysis of function. Am J Sports Med. 1988; 16:97Y Blazar PE, Iannotti JP, Williams GR. Anterior posterior instability of the distal clavicle after distal clavicle resection. Clin Orthop. 1998;348:114Y Pratt NE. Anatomy and biomechanics of the shoulder. J Hand Ther. 1994;7:65Y Moore KL, Dalley AF. Clinically Orientated Anatomy. 4th ed. Baltimore, MD: Lippincott Williams & Wilkins; Lindsey RW, Gutowski WT. The migration of a broken pin following fixation of the acromioclavicular joint. Orthopedics. 1986;9:413Y Berkowitz MM, Warren RF, Altchek DW, et al. Arthroscopic acromioclavicular resection. Oper Tech Sports Med. 1997;5(2):60Y64. 84