Distal clavicle fractures are rare and account for approximately 15% to 25% of clavicle fractures¹. Recently, internal fixation using a hook plate has been introduced for distal clavicle fractures, but complications such as delayed union, nonunion, and acromion osteolysis have been reported^2-4. The conventional AO hook locking compression plate (LCP; Synthes) is not sufficient to fix comminuted fragments or small fragments because it has only three holes for lateral fragments. Especially in Neer type IIb distal clavicle fractures, it can cause loss of coracoclavicular distance restoration. A novel hybrid hook LCP (TDM Corp.) was designed to overcome these limitations⁵. It has eight 2.7-mm divergent angle lateral holes for locking screws.

We report a case of regeneration of acromion osteolysis after novel hybrid hook LCP fixation for distal clavicle fractures. To our knowledge, this case report represents the first description of the regeneration of acromion after osteolysis.

This retrospective report was approved by the Institutional Review Board of the Dankook University Hospital (No. DKUH 2020-02-013).

A 60-year-old right-hand-dominant man was involved in a motorcycle driver’s accident, resulting in a Neer type IIb distal clavicle fracture of his left shoulder (Fig. 1). He had no other combined injuries or underlying diseases. He underwent open reduction and internal fixation with a novel hybrid hook LCP 6 days after the injury (Fig. 2). No procedure was undertaken for the coracoclavicular ligament in this case. Shoulder immobilizer was used for 6 weeks after surgery. Only pendulum exercises and assisted passive mobilization were received during the period with the shoulder immobilizer. Six weeks after surgery, the patient was allowed to progressively increase the range of motion and pain-free strengthening exercises. He visited the outpatient consultation unit 3 weeks, 6 weeks, and 12 weeks after surgery. During his postoperative visits, he did not complain of any specific symptoms, and no specific findings were observed on follow-up radiographs.

Five months after internal fixation with a hook plate, the patient underwent surgery for the removal of the hook plate. Radiographs after removal of the hook plate showed erosion of the acromion (Fig. 3). His American Shoulder and Elbow Surgeons (ASES) shoulder score was 82 and his Korean Shoulder Scoring (KSS) System score was 75, and he complained of pain of intensity 5 on the visual analogue scale (VAS) with persistent tenderness of the acromion. We decided to perform a conservative treatment without any immobilization for acromion osteolysis and observed the progress. A final radiograph that was obtained 10 months after hook plate removal shows regeneration of the acromion (Fig. 4). He returned fully to daily activities, and a complete range of shoulder motion (forward elevation, 165°; external rotation, 86°; and internal ration, L1) was achieved. His ASES score was 90 and KSS was 83. However, he complained of pain of intensity 3 on the VAS, and persistent tenderness of the acromion.

Distal clavicle fractures account for approximately 15% to 25% of all clavicle fractures and are subdivided by the attachment of the coracoclavicular ligament and clavicle¹. Neer type-II distal clavicle fractures are unstable because two forces act at the fracture site; the superior force exerted by the trapezius muscle acting on the medial fragment, and the descending force exerted by the deltoid muscle acts on the lateral fragment^6-8. Therefore, rates of delayed union or nonunion are high with conservative treatment in this type of clavicle fracture^6,7,9. Therefore, surgical fixation is recommended for Neer type-II distal clavicle fractures^6-8.

Various surgical techniques have been introduced previously, such as tension band wiring, intramedullary pinning, locking plates, and hook plates for distal clavicle fractures^5-10. However, no consensus has been achieved on the gold standard for unstable distal clavicle fractures. Recently, hook plate fixation for unstable distal clavicle fractures has become increasingly popular^5,6,8. Hook plates can allow relative ease of implant insertion and accurate maintenance of reduction. In addition, hook plate fixation allows some motion between the bones, which most closely reflects the biomechanics of the native acromioclavicular joint. This means that it is safe to be physiologically flexible while providing reliable immobilization. The procedure may be accompanied by repair or reconstruction of the coracoclavicular ligament in Neer type IIb distal clavicle fractures⁷. Although hardware removal is typically optional for those with conventional plates, a high percentage of patients with hook plate fixation will require plate removal to regain full range of shoulder motion and avoid acromioclavicular joint arthritis and subacromial impingement^1,5,6,9.

With the increasing use of hook plates, various complications have been reported. Among them, the lateral part of the hook plate engaging below the acromion can cause subacromial impingement^2-4,9. The increasing stress on the acromion may lead to osteolysis and fracture of the acromion. There can be various factors affecting osteolysis of the acromion such as retention time of the implant, and placement of the hook.

In our case, the patient underwent hook plate fixation for type IIb distal clavicle fracture. We removed the hook plate 5 months after the initial surgery. The radiograph taken after the removal showed acromial erosion. For acromial osteolysis, we performed conservative treatment, and regeneration of the acromion was observed on radiographs 10 months after hook removal. In addition, the patient’s functional outcome was favorable, although there remains a slight pain. We believe that the short length of the hook in the novel hybrid hook plate may cause severe stress concentration, leading to the potential induction of osteolysis, and the acromion regenerates because the periosteum remains even if erosion caused by the hook plate occurs.

We presented a clinical case of the unexpected complication of acromial osteolysis with subsequent regeneration of the acromion. We thought that the periosteum played a significant role in the acromion regeneration despite no special treatment.