Anthropometry of the Coracoid Process: Surgical Considerations in Arthroscopic Coracoclavicular Ligament Reconstruction

Purposed: The purpose of this study was to measure the dimensions of the coracoid process from MRI imaging to nd a safe area for drilling in arthroscopic coracoclavicular ligament reconstruction. Materials and methods: A retrospective study of patients who underwent MRI of the shoulder in our hospital between July 1, 2016 and June 31, 2018. Inclusion criteria included patients aged 20–60 years, BMI < 35 and height > 150 cm. Exclusion criteria included patients with a history of coracoid surgery, coracoid fracture or bone diseases affecting the coracoid bone. Measurement of the coracoid size was done in T1 MRI sagittal, coronal and axial views. Safe zone was dened as an area far from the medial and lateral base of the coracoid process, at least 5 mm each. Safe zone was then compared with the size of the drill hole to nd an appropriately sized drill that would not violate the peripheral cortical coracoid bone. Results: Overall, 100 (male = 55, female = 45) patients were included in this study. Mean age, BMI and height of the patients were 48.5 years (range 22–58), 22.5 kg/m 2 (range 18.5–28.4) and 164.5 cm (range 155–182), respectively. Mean safe area was 167.45 mm 2 with males being slightly larger than females (male = 190.34 mm 2 , female = 140.99 mm 2 ). The drill hole area was calculated by the radius (r) of the drill ( compared with the safe area. The maximum drill size for the men was 7 mm (154 mm 2 ) and 6 mm (113.14 mm 2 ) for women. However, we recommended a smaller drill size to account for any errors in position and technique. Conclusion: The safe area at the coracoid base was slightly larger in males as compared to females and the proper drill bit size was suggested to be less than 7 mm in males and 6 mm in females.

Introduction Acromioclavicular (AC) joint (ACJ) separation is a common injury that can result from sports activities and direct impact to the lateral aspect of the shoulder.Reconstruction of the coracoclavicular ligament may improve biomechanical function and clinical outcome. (1) Arthroscopic coracoclavicular ligament reconstruction with cortical suspension at the coracoid base has bene t in the treatment of ACJ injuries due to its better visualization, identi cation of occult intra-articular pathologies and less chance of soft tissue injuries when compare with open surgery. Nevertheless, complications such as persistent instability, loss of reduction, coracoid fracture, slip of the coracoid button with subsequent recurrent vertical instability and wound infection were observed.(2)One of the potential risks of this procedure is fracture of the coracoid process from the improper positioning and sizing of the drill hole(3), which leads to failure of treatment and the increased morbidity of the patient.
The purpose of this study was to measure the dimensions of the coracoid process from MRI to identify a safe area for drilling. We hypothesized that safe area for coracoid drilling was smaller in female compared to that in male.
Measurements of the coracoid size were done in T1 MRI sagittal, coronal and axial views by one orthopedic resident and one radiologist. Tip width, base width and length were measured in a T1 MRI axial view (Fig. 1). Tip height and base height were measured in a T1 MRI sagittal view (Fig. 2). Insertion of the trapezoid and conoid ligaments was identi ed in T1 MRI axial and coronal views. Measurements were repeated three times with a 2 week interval.Intra-observer and inter-observer reliability were calculated withIntraclass Correlation Coe cients (ICC), 0.86 and 0.65 respectively.
Safe zone was de ned as an area far from the medial and lateral coracoid border at least 5 mm each. (4) The upper border included the insertion of the trapezoid ligament, and the lower border included the coracoid base. We measured the safe area by formula area of the trapezoid (Fig. 3). The safe zone was then compared with the size of the drill hole to nd an appropriately sized drill that would not violate the peripheral cortical coracoid bone.
Sample size was calculated based on measurements of the coracoid process from a previous study. (5) To achieve an alpha of 0.05 and beta of 0.20 with 80% power, it required 50 patients.

Statistical analyses included means with standard deviations and unpaired t-tests.Statistical analyses
were performed with SPSS (Statistical Package of Social Sciences, Chicago, IL, USA). The level of statistical signi cance was set at a P-value < 0.05.

Results
All demographic data are illustrated in Table 1. There were no signi cant differences noted between either of the groups except for height (P < 0.0001) The coracoid measurements are illustrated in Table 2. There were signi cant differences between both groups in terms of length (P < 0.0001), base width (P < 0.0001) and tip width (P = 0.02).
The safe area measurement is illustrated in Table 3. There were signi cant differences between both of the groups (P < 0.0001).

Discussion
Nowadays, many techniques for the restoration of coracoclavicular ligaments have been reported with improved biomechanical function and clinical outcomes.(6) In transclavicular-transcoracoidal tunnel procedures, accurate placement may reduce the risk of repair failure and cortical breach, as described in recent anatomic considerations for transclavicular-transcoracoid drilling for coracoclavicular ligaments.  10) reported a mean for the distance from the tip to trapezoid ligament of 3.33 cm and a mean coracoid length, tip width and tip height of 4.26, 1.49 and 2.11 cm, respectively.As expected, men had a signi cantly longer coracoid, greater coracoid tip width and height and a greater coracoid base width and height than women.(7-10) The coracoid processes in our study were smaller when compared to previous studies, which may be due to the ethnicity of the participants.(7-10) The mean safe area was 167.45 mm 2 with males being slightly larger than females (male = 190.34 mm 2 , female = 140.99 mm 2 ). The drill hole area was calculated by the radius (r) of the drill ( compared with the safe area. The maximum drill size for men was size 7 mm (154 mm 2 ) and 6 mm (113.14 mm 2 ) for women. However, we recommend a smaller drill size to account any errors in positioning and technique.

Conclusion
The safe area at the coracoid base was slightly larger in males as compared to females and a proper drill bit size is suggested to be less than 7 mm in males and 6 mm in females.

Declarations Compliance with Ethical Standards
Con ict of Interest: The authors declare that they have no con ict of interest.
Funding: There is no funding source. Ethical approval:The study was approved by Institutional Review Board Faculty of Medicine Chulalongkorn University, IRB No.307/62 acromioclavicular joint dislocations using 2 TightRope devices: a biomechanical study. Am J Sports Med 36 (12)

Figure 3
The green area was the safe area generated from the formula area of the trapezoid ½(A+B)(H), A: Insertion of the trapezoid ligament, B: Coracoid base, H: Distance between the trapezoid ligament insertion and coracoid base and *: "Error in tunnel placement" de ned as 5 mm from the medial and lateral coracoid border.