Implant removal after clavicle plating is common. Low-profile dual mini-fragment plate constructs are considered safe for fixation of diaphyseal clavicle fractures. The aim of this study was to investigate: (1) the biomechanical competence of different dual plate designs from stiffness and cycles to failure, and (2) to compare them against 3.5mm single superoanterior plating. Twelve artificial clavicles were assigned to 2 groups and instrumented with titanium matrix mandible plates as follows: group 1 (G1) (2.5mm anterior+2.0mm superior) and group 2 (G2) (2.0mm anterior+2.0mm superior). An unstable clavicle shaft fracture (AO/OTA15.2C) was simulated. Specimens were cyclically tested to failure under craniocaudal cantilever bending, superimposed with torsion around the shaft axis and compared to previous published data of 6 locked superoanterior plates tested under the same conditions (G3). Displacement (mm) after 5000 cycles was highest in G3 (10.7±0.8) followed by G2 (8.5±1.0) and G1 (7.5±1.0), respectively. Both outcomes were significantly higher in G3 as compared to both G1 and G2 (p≤0.027). Cycles to failure were highest in G3 (19536±3586) followed by G1 (15834±3492) and G2 (11104±3177), being significantly higher in G3 compared to G2 (p=0.004). Failure was breakage of one or two plates at the level of the osteotomy in all specimens. One G1 specimen demonstrated failure of the anterior plate. Both plates in other G1 specimens. Majority of G2 had fractures in both plates. No screw pullout or additional clavicle fractures were observed among specimens. Low-profile 2.0/2.0 dual plates demonstrated similar initial stiffness compared to 3.5mm single plates, however, had significantly lower failure endurance. Low-profile 2.5/2.0 dual plates showed significant higher initial stiffness and similar resistance to failure compared to 3.5mm single locked plates and can be considered as a useful alternative for diaphyseal clavicle fracture fixation. These results complement the promising results of several clinical studies

Recently, a new generation of superior clavicle plates was developed featuring the variable-angle locking technology for enhanced screw positioning and optimized plate-to-bone fit design. On the other hand, mini-fragment plates used in dual plating mode have demonstrated promising clinical results. However, these two bone-implant constructs have not been investigated biomechanically in a human cadaveric model. Therefore, the aim of the current study was to compare the biomechanical competence of single superior plating using the new generation plate versus dual plating with low-profile mini-fragment plates. Sixteen paired human cadaveric clavicles were assigned pairwise to two groups for instrumentation with either a 2.7 mm Variable Angle Locking Compression Plate placed superiorly (Group 1), or with one 2.5 mm anterior plate combined with one 2.0 mm superior matrix mandible plate (Group 2). An unstable clavicle shaft fracture AO/OTA15.2C was simulated by means of a 5 mm osteotomy gap. All specimens were cyclically tested to failure under craniocaudal cantilever bending, superimposed with bidirectional torsion around the shaft axis and monitored via motion tracking. Initial stiffness was significantly higher in Group 2 (9.28±4.40 N/mm) compared to Group 1 (3.68±1.08 N/mm), p=0.003. The amplitudes of interfragmentary motions in terms of craniocaudal and shear displacement, fracture gap opening and torsion were significantly bigger over the course of 12500 cycles in Group 1 compared to Group 2; p≤0.038. Cycles to 2 mm shear displacement were significantly lower in Group 1 (22792±4346) compared to Group 2 (27437±1877), p=0.047. From a biomechanical perspective, low-profile 2.5/2.0 dual plates demonstrated significantly higher initial stiffness, less interfragmentary movements, and higher resistance to failure compared to 2.7 single superior variable-angle locking plates and can therefore be considered as a useful alternative for diaphyseal clavicle fracture fixation especially in unstable fracture configurations

Background. Clavicle development occurs before the age of 9 in females and 12 in males. Children below the age of 10 with displaced midshaft clavicle fractures recover well with conservative management. However adolescents are more demanding of function and satisfaction following clavicle fractures and may benefit from operative management. Study aims: 1) Perform a systematic review of the current evidence supporting intramedullary fixation of adolescent clavicle fractures. 2) Review current management in a major trauma center (MTC) with a view to assess feasibility for a randomised controlled trial (RCT). Methods. The MEDLINE, EMBASE and AMED databases were searched in October 2014 to identify all English language studies evaluating intramedullary fixation in children aged 10–18 years using MeSH terms. Data was extracted using a standardised data collection sheet and studies were critically appraised by aid of the PRISMA checklist. All patients aged 9–15 attending an MTC receiving clavicle radiographs in 2014 were retrospectively reviewed for type of fracture, management and outcome. Results. Literature search identified 54 articles. After application of exclusion criteria 3 studies were selected for final review. 47 adolescent patients received intramedullary clavicle fixation from a prospective and two retrospective case series. 61 adolescents presented to our unit with a clavicle fracture in 2014, 2 were lost to follow-up, 54 were managed non-operatively, 3 received titanium-elastic nailing, 1 plate osteosynthesis and 1 bone suture. 0 and 19 patients reported a palpable lump, mean time to pain resolution was 4 and 6 weeks and time to full range of motion was 4 and 5 weeks following operative and conservative management respectively. All patients reached radiographic union. Conclusion. Current evidence supporting intramedullary fixation of clavicle fractures in adolescents is poor. There remains clinical equipoise on the best management of these patients, however they are predominantly treated conservatively. A future multi-center RCT may be feasible. Level of Evidence. 1

Abstract. Objectives. Clavicle fractures are common, yet debate exists regarding which patients would benefit from conservative versus operative management. Traditionally shortening greater than 2cm has been accepted as an indicator for surgery. However, clavicle length varies between individuals. In a cadaveric study clavicle shortening greater than 15% was suggested to affect outcomes. There is no clinical correlation of this in the literature. In this study we investigate outcomes following middle third clavicle fractures and the effect of percentage shortening on union rates. Methods. We identified a consecutive series of adults with primary midshaft clavicle fractures presenting to our institution from April 2015-March 2017. Clinical records and radiographs were reviewed to elicit outcomes. Time to union was measured against factors including; percentage shortening, displacement, comminution and smoking. Statistical significance was calculated. Results. 127 patients were identified, of whom 90 were managed conservatively and 37 operatively. Fractures were displaced in 86 patients (68%). Mean age was 41.7 years (range 18–89). Mean time to union for displaced fractures was longer than for undisplaced at 13.4 and 8.9 weeks respectively (p=0.0948). Displaced fractures treated operatively had mean time to union of 12.8 weeks, three weeks shorter than those managed conservatively (p=0.0470). Mean time to union for fractures with >15% shortening was 16.0 weeks, nearly double the 8.7 weeks with <15% shortening (p= 0.0241). Smokers had 8 weeks longer time to union (p=0.0082). Nonunion rate was 10% in fractures managed conservatively and 0% in those treated operatively. Complications following operative management were plate removal (13.5%), frozen shoulder (8.1%) and infection (2.9%). Conclusions. Nonunion rate is higher in fractures managed conservatively. Shortening >15% leads to significantly longer union time and should therefore be used as an indicator for surgery. Displacement and smoking also lengthen time to union and should be considered in the operative decision process. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Last decade, a shift towards operative treatment of midshaft clavicle fractures has been observed [T. Huttunen et al., Injury, 2013]. Current fracture fixation plates are however suboptimal, leading to reoperation rates up to 53% [J. G. Wijdicks et al., Arch. Orthop. Trauma Surg, 2012]. Plate irritation, potentially caused by a bad geometric fit and plate prominence, has been found to be the most important factor for reoperation [B. D. Ashman et a.l, Injury, 2014]. Therefore, thin plate implants that do not interfere with muscle attachment sites (MAS) would be beneficial in reducing plate irritation. However, little is known about the clavicle MAS variation. The goal of this study was therefore to assess their variability by morphing the MAS to an average clavicle. 14 Cadaveric clavicles were dissected by a medical doctor (MH), laser scanned (Nikon, LC60dx) and a photogrammetry was created with Agisoft photoscan (Agisoft, Russia). Subsequently a CT-scan of these bones was acquired and segmented in Mimics (Materialise, Belgium). The segmented bone was aligned with the laser scan and MAS were indicated in 3-matic (Materialise, Belgium). Next, a statistical shape model (SSM) of the 14 segmented clavicles was created. The average clavicle from the SSM was then registered to all original clavicle meshes. This registration assures correspondences between source and target mesh. Hence, MAS of individual muscles of all 14 bones were indicated on the average clavicle. Mean area is 602 mm. 2. ± 137 mm. 2. for the deltoid muscle, 1022 mm. 2. ±207 mm. 2. for the trapezius muscle, and 683 mm. 2. ± 132 mm. 2. for the pectoralis major muscle. The sternocleidomastoid muscle has a mean area of 513 mm. 2. ± 190 mm. 2. and the subclavius muscle had the smallest mean area of 451 mm. 2. ± 162 mm. 2. Visualization of all MAS on the average clavicle resulted in 72% coverage of the surface, visualizing only each muscle's largest MAS led to 52% coverage. The large differences in MAS surface areas, as shown by the standard deviation, already indicate their variability. Difference between coverage by all MAS and only the largest, shows that MAS location varies strongly as well. Therefore, design of generic plates that do not interfere with individual MAS is challenging. Hence, patient-specific clavicle fracture fixation plates should be considered to minimally interfere with MAS

Primary bone tumours of the clavicle are rare. Currently the existing literature is limited to a single case series and case reports or cases. Information regarding the patient's demographics and tumour types is therefore limited. The aim of this study was to investigate the and also suggest a management protocol for suspected primary bone tumours of the clavicle. We retrospectively reviewed the Scottish Bone Tumour Register from January 1971 to January 2012 and included all primary bone tumours of the clavicle. We identified only sixteen primary bone tumours over forty one year's highlighting the rarity of these tumours. There were ten benign and six malignant tumours with a mean age of 32 years (Range 4 to 66). The average presentation to orthopaedics after onset of symptoms was two months with five patients presenting following a pathological fracture. Malignant tumour types identified were consistent with previous literature with two cases of Ewing's sarcoma and osteosarcoma and a single case of osteosarcoma post radiotherapy and a single case of chondrosarcoma. Benign tumours were treated effectively with intralesional procedures. Malignant tumours were treated with wide local excision and subtotal or total clavicle excision. We suggest an investigatory and treatment protocol for patients with a suspected primary bone tumour of the clavicle. This is the largest series of primary bone tumours of the clavicle in the literature

Background. Open reduction and internal fixation using plate osteosynthesis for midshaft clavicle fractures is often associated with hardware prominance. Although clinical studies have suggested a role for the use of thinner 2.7mm plates as a means of increasing cosmetic acceptability this still remains an area of controversy. We investigated the effect of plate size (2.7mm vs. 3.5mm), plate treatment (annealed vs. cold worked) and number of screws on the stiffness and yield point. Methods. Twenty-four synthetic clavicles were randomly divided into four treatment groups - Synthes (Synthes, Paoli, PA) 2.7mm cold-worked calcaneal reconstruction plate with six or eight bicortical screws; 3.5mm LCP reconstruction plate (RP) and 3.5mm LCP pre-contoured superior-anterior clavicle plate (PCSA). After measuring the baseline stiffness of the intact specimens, all clavicles were plated, a wedge-shaped inferior defect was created and testing performed using a cantilever-bending model. Statistical analysis was performed using one-way ANOVA with Tukey's multiple comparison test with significance set at a P value <0.05. Results. The 3.5mm RP construct was significantly stiffer than both of the 2.7mm CR constructs (P < 0.0001). The yield point for the 3.5mm PCSA construct was greater than the other three constructs (P < 0.0001), while the yield point for the 2.7mm CR plate with 6 screws and with 8 screws was higher than the 3.5mm RP construct (P = 0.0002 and P = 0.0023 respectively). The amount of displacement required to reach the yield point was highest for the 2.7mm CR plate with six screws and this was significantly higher than the values for the other three constructs. Conclusion. The 3.5mm plates demonstrated increased bending stiffness compared to the 2.7mm plates. Despite the lower resistance to bending forces, the cold worked 2.7mm plate exhibited a significantly higher yield point and required significantly more superior to inferior displacement to deform

In an osteological collection of 3100 specimens, 70 were found with unilateral clavicular fractures which were matched with 70 randomly selected normal specimens. This formed the basis of a study of the incidence of arthritis of the acromioclavicular joint and the effect of clavicular fracture on the development of arthritis in the ipsilateral acromioclavicular joint. This was graded visually on a severity scale of 0 to 3. The incidence of moderate to severe arthritis of the acromioclavicular joint in normal specimens was 77% (100 specimens). In those with a clavicular fracture, 66 of 70 (94%) had arthritis of the acromioclavicular joint, compared to 63 of 70 (90%) on the non-injured contralateral side (p = 0.35).

Clavicles with shortening of 15 mm or less had no difference in the incidence of arthritis compared to those with shortening greater than 15 mm (p = 0.25). The location of the fracture had no effect on the development of arthritis.

The periclavicular space is a conduit for the brachial plexus and subclavian-axillary vascular system. Changes in its shape/form generated by alteration in the anatomy of its bounding structures, e.g. clavicle malunion, cause distortion of the containing structures, particularly during arm motion, leading to syndromes of thoracic outlet stenosis etc., or alterations of scapular posture with potential reduction in shoulder function. Aim of this study was developing an in vitro methodology for systematic and repeatable measurements of the clinically poorly characterized periclavicular space during arm motion using CT-imaging and computer-aided 3D-methodologies. A radiolucent frame, mountable to the CT-table, was constructed to fix an upper torso in an upright position with the shoulder joint lying in the isocentre. The centrally osteotomized humerus is fixed to a semi-circular bracket mounted centrally at the end of the frame. All arm movements (ante-/retroversion, abduction/elevation, in-/external rotation) can be set and scanned in a defined and reproducible manner. Clavicle fractures healed in malposition can be simulated by osteotomy and fixation using a titanium/carbon external fixator. During image processing the first rib served as fixed reference in space. Clavicle, scapula and humerus were registered, segmented, and triangulated. The different positions were displayed as superimposed surface meshes and measurements performed automatically. Initial results of an intact shoulder girdle demonstrated that different arm positions including ante-/retroversion and abduction/elevation resulted solely in a transverse movement of the clavicle along/parallel to the first rib maintaining the periclavicular space. A radiolucent frame enabling systematic and reproducible CT scanning of upper torsos in various arm movements was developed and utilized to characterize the effect on the 3D volume of the periclavicular space. Initial results demonstrated exclusively transverse movement of the clavicle along/parallel to the first rib maintaining the periclavicular space during arm positions within a physiological range of motion

Osteoarthritis (OA) is the most common disorder of the Sternoclavicular Joint (SCJ). In our case-control study, we evaluated the relationship between clavicular length and OA at the SCJ. CT scans of adults presenting to the Emergency Department of our hospital were examined to look for OA, defined as the presence of osteophytes, subchondral cysts, or cortical sclerosis at the SCJ. Medial-most and lateral-most points of the clavicle were marked on the slices passing through the SC and AC joints respectively. Using x, y, and z-axis coordinates from the DICOM metadata, clavicular length was calculated as the distance between these two points with 3D geometry. Preliminary data of 334 SCJs from 167 patients (64% males, 36% females) with a mean age of 48.5 ± 20.5 years were analysed. Multivariate regression models revealed that age and clavicular length were independent risk factors for OA while gender did not reach statistical significance. A 1mm increase in length was associated with 9% and 7% reduction in the odds of developing OA on the left and the right respectively. Comparing the mean clavicular length using t-test showed a significantly shorter clavicle in the group with OA (145.8 vs 152.7, p=0.0001, left and 144.2 vs 150.3, p=0.0007, right). Our data suggest that the risk of developing OA at the SCJ is higher for shorter clavicles. This could be of clinical relevance in cases of clavicular fracture where clavicular shortening might lead to a higher risk of developing OA. Biomechanical studies are needed to find out the mechanism of this effect

Abstract. Objectives. We aimed to evaluate if union of clavicle fractures can be predicted at six weeks post-injury by the presence of bridging callus detected by ultrasound. Methods. Adult patients who sustained a displaced midshaft clavicle were recruited prospectively. We assessed patient demographics, functional scores and radiographic predictors with a standardized protocol at six weeks. Ultrasound evaluation of the fracture site was undertaken to determine if sonographic bridging callus was present. Nonunion was determined by CT scanning at six months post-injury. Clinical features at six weeks were used to stratify patients at high risk of nonunion and a QuickDASH ≥40, fracture movement on examination or absence of callus on radiograph. Results. 112 patients completed follow-up at six months with a nonunion incidence of 17% (n=18/112). Sonographic bridging callus was detected in 62.5% (n=70/112) of the cohort at six weeks post-injury. If present, union occurred in 98.6% of the fractures (n=69/70). If absent, nonunion developed in 40.5% of cases (n=17/42). The sensitivity to predict union with sonographic bridging callus at six weeks was 73.4% and the specificity was 94.4%. Regression analysis found failure to detect sonographic bridging callus at six weeks was associated with nonunion, older age, female sex, and greater overall fracture displacement (Nagelkerke R2=0.60). Of the cohort, 30.4% (n=34/112) had absent sonographic bridging callus at six weeks in combination with one or more of the ‘high risk’ clinical features. If one was present the nonunion rate was 47.1%, increasing to 60% with two risk factors and 100% when combined with all three. Conclusions. Ultrasound can accurately predict fracture healing at six weeks following a displaced midshaft clavicle fracture. When combined with poor clinical recovery this could be used to target patients for early operative intervention. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Purpose. Clavicular anatomy is variable and this has implications when osteosynthesis is undertaken especially while using the newer generation pre-contoured anatomical plates. This study aimed to examine the anatomy of the clavicle and its variations. Methods. One hundred and forty three clavicles of unknown origin were analysed. Photographs were taken of each clavicle. A scale and electronic callipers were used to determine specific dimensions from six predefined areas of the bone. The length, diameters of the acromial and sternal heads, the diameter of the mid-clavicular segment and depths of both the medial and lateral curvatures were obtained. Results. Clavicles studied ranged from 18.34 centimetres to 10.93 centimetres in length, with the mean length being 14.97 centimetres thus, demonstrating 7.41±1.38 centimetres variability in length between extremes of the clavicles measured. In general, left clavicles were found to be longer than right clavicles. Mid-clavicular segments showed a greater diameter on the left than right, with 1.07±0.17 centimetres difference between the extremes of diameters. The mean diameter of the sternal and acromial heads showed little variation at 2.36±0.38 centimetres and 2.54±0.47 centimetres respectively. Medial curvatures were found to be deeper than lateral curves (1.87±0.36 centimetres vs. 1.28±0.46 centimetres), although the lateral curves showed more bone to bone variation. Conclusion. The results of this study define the dimensions of the clavicle and illustrate the wide variations in its anatomy. This variability shown in large group of cadaveric specimens would be immensely helpful to consider while designing the pre-contoured anatomical plates. In addition, surgeons should note these variations in the clavicular anatomy while undertaking osteosynthesis as these variations have the potential of causing a mismatch between the newer pre-contoured plates and the fractured clavicle fixation

Summary. Analysis of existing data of patients who had undergone debridement procedure for osteoarthritis (OA) of glenohumeral joint showed improvement in disability at a minimum of one year following surgery. Injured workers were significantly younger and had a poorer outcome. Introduction. There is little information on debridement for OA of the shoulder joint. The purpose of this study was to examine factors that affect the outcome of arthroscopic debridement with or without acromioplasty /resection of clavicle of patients with osteoarthritis of the glenohumeral joint, in subjective perception of disability and functional range of motion and strength at a minimum of one year following surgery. Patients and Methods. Existing data of patients with advanced OA of the glenohumeral joint who had undergone debridement were used for analysis. These patients were not good candidates for shoulder arthroplasty due to a young age, high activity level, or desire to avoid major surgery at the time of assessment. Arthroscopic debridement included removal of loose bodies, chondral flaps, and degenerative tissue. Resection of the lateral end of the clavicle or acromioplasty was performed as clinically indicated for management of osteoarthritis of the Acromioclavicular (AC) joint or subacromial impingement respectively. Disability at a minimum of 12 months following surgery was measured by the American Shoulder and Elbow Surgeon's (ASES) assessment form, Constant-Murley score (CMS), strength, and painfree range of motion (ROM) in four directions. Impact of sex, age, having acromioplasty or resection of clavicle, and having an active work-related compensation claim was examined. Results. Seventy-four patients (mean age= 55, SD: 14 (range: 25–88), range: 35–86, 34 females, 40 males) were included in analysis. The average symptom duration was 5.8 years. Fifty nine (80%) patients had an associated subacromial decompression [55 (74%) had acromioplasty, 32(43%) had resection of the lateral end of the clavicle, and 28 (38%) had both procedures]. Nineteen (26%) patients had a work-related compensation claim related to their shoulder. This group was significantly younger than the non-compensation group (45 vs. 58, p=0.0001). Paired student t-tests showed a statistically significant improvement in scores of ASES and CMS (p<0.0001), strength (p=0.001) and painfree range of motion (p=0.01) at a minimum of 1 year follow-up. The ANCOVA model that incorporated sex, age, additional decompression (AC resection or acromioplasty), compensation claim and pre-op scores, showed that the pre-op scores and having a work-related claim were the most influential predictors of post-op scores of ASES, CMS, and ROM. The post-op strength was the only factor that was affected by sex, age and having a work-related claim. Discussion/Conclusion. Arthroscopic debridement with or without acromioplasty /resection of the lateral end of the clavicle improved disability, painfree range of motion and strength in patients suffering from osteoarthritis of glenohumeral joint at a minimum of one year following surgery. Patients with an active compensation claim related to their shoulder were significantly younger and had a poorer outcome

Tightrope fixation is known method for reconstructing acromioclavicular joint and the presence of good bone stock around the two drillholes is the most important determining factor for preventing failure. Aim. Arthroscopic-assisted tightrope stabilisation involve drilling clavicle and coracoids in a straight line. This leads to eccentric drillholes with inadequate bone around it. Open tightrope fixation involves drilling holes under direct vision, independently and leading to centric hole with adequate bone around it. Our study assesses the hypothesis of tightrope fixation in relation to location of drillholes using CT-scan and cadaveric models for arthroscopic and open technique for ACJ fixation. Methods. CT-scans of 20 shoulders performed. Special software used to draw straight line from distal end of clavicle to coracoid. Bone volume around coracoid drillhole was calculated. Cadaveric shoulder specimens were dissected. The arthroscopic technique was performed under vision by drilling both clavicle and base of coracoid holes in one direction. Same specimens were used for open technique. Base of coracoid crossectioned and volume calculated. Results. 40 shoulders were included(20 cadaveric specimens&20 CT). Bone stock was adequate in both techniques. Variable angle for insertion of drillholes using arthroscopic technique were needed depending on shape of shoulder. Conclusion. Tightrope allows nonrigid anatomic fixation of acromioclavicular joint. Published studies showed high rate of fixation failure with tightrope system but with patient satisfaction and high functional results. Our study showed adequate bone stock around coracoid in both open and arthroscopic technique. Mode of failure remains unclear and we recommend further biomechanical studies to assess failure factors

We studied the origin of the anterior deltoid from the lateral third of the clavicle and the leading anterior edge of the acromion in 18 cadaver shoulders by anatomical and histological methods. The main origin of the deltoid was from the superior surface of the anterior acromion, but muscle and tendinous attachments were also seen on the entire anterior surface of the acromion, its anteroinferior surface and on the whole width of the anterior surface of the clavicle. Mock arthroscopic acromioplasty was shown to detach deltoid fibres from the anterior surfaces, leaving the superior attachment in continuity. Potentially, arthroscopic subacromial and clavicular resection can detach deltoid fibres originating from the anterior and anteroinferior surfaces of the acromion and clavicle and thus weaken the anterior deltoid

Summary Statement. Reverse shoulder design philosophy can impact external rotation moment arms. Lateralizing the humerus can increase the external rotator moment arms relative to normal anatomy. Introduction. The design of reverse shoulders continues to evolve. These devices are unique in that they are not meant to reproduce the healthy anatomy. The reversal of the fulcurm in these devices impacts every muscle that surrounds the joint. This study is focused on analyzing the moment arms for the rotator cuff muscles involved in internal and external rotation for a number of reverse shoulder design philosophies. Methods. Four of the most common design philosophies were chosen. The first, a Grammont style prosthesis, with a center of rotation (COR) on the glenoid face and a humeral cup countersunk into the proximal humerus (MGMH). The second concept is the MGMH design lateralised by a 10mm bone graft (BIO). The third concept has a lateralised glenosphere COR and a humeral component inside the proximal humerus (LGMH). The fourth design has a medialised COR with a humeral component placed on top of the humerus (MGLH). This places the humerus further lateral than the previous designs. For each component set, a representative implant was modeled based on published specifications. Each design was implanted into the same digital bone models (consisting of a humerus, scapula, clavicle, and ribcage) following the manufacturer's recommended surgical technique. The muscles analyzed were the posterior-deltoid (PD), subscapularis (SSC), infraspinatus (IS), and teres minor (TM). These muscles were allowed to wrap around the bone of the scapula and proximal humerus through the range of motion. All muscle origin and insertion points were kept constant throughout the analysis. The assemblies were externally rotated from an initial position of 45° internal rotation to 45° of external rotation of the humerus with the arm at 0° of abduction. The moment arms for all muscles were compared to those calculated for the anatomic shoulder. Results. All the rotator cuff muscles displayed a similar trend with the reverse shoulder. The external rotators all had similar moment arm values at neutral (IS∼22mm, TM∼20mm), but increased at rates proportional to their humeral offsets with external rotation (IS-MGLH 32.3mm, LGMH 27.5mm, MGMH and BIO 26.25mm; TM-MGLH 31.3mm, LGMH 27.8mm, MGMH and BIO 26.5mm). The SSC internal rotation moment arm remains roughly constant at 20mm for the anatomic shoulder, but varies widely from 45° external to 45° internal rotation with the different designs (MGLH 31.4mm to 6.7mm; MGMH 25.1mm to 11.2mm; LGMH 26.2mm to 10.8mm; BIO 25.4mm to 4.8mm). The PD moment arm is increased relative to the anatomic shoulder during external rotation for the MGLH design (9.3mm vs. 7.4mm). The other designs exhibit a decrease in the moment arm of this muscle relative to the anatomic design (LGMH 7.3mm, MGMH 5.8mm, BIO 6.4mm). Discussion. The lateral offset between the center of humeral axis and the muscle insertion on the humerus dominates the external rotation moment arm value through this range of motion. This is evident by the increase in the moment arms with external rotation for the different reverse shoulder designs. The increase in external rotation efficiency for the external rotators and PD could play a critical role in post-operative external rotation strength and motion

The aim of this study was to establish a classification system for the acromioclavicular joint using cadaveric dissection and radiological analyses of both reformatted computed tomographic scans and conventional radiographs centred on the joint. This classification should be useful for planning arthroscopic procedures or introducing a needle and in prospective studies of biomechanical stresses across the joint which may be associated with the development of joint pathology.

We have demonstrated three main three-dimensional morphological groups namely flat, oblique and curved, on both cadaveric examination and radiological assessment. These groups were recognised in both the coronal and axial planes and were independent of age.