1 Introduction

Acromioclavicular (AC) joint ligament injuries are found in 9% of all shoulder trauma and some cases require surgical intervention (Rockwood type III and above) in order to minimize functional loss.¹ Clavicle hook plates have been found to be an effective method to fix AC joint dislocation. Subacromial hook positioning allows the plate to act as a depressor, bringing the clavicle down to maintain AC joint position.² Although excellent clinical outcomes have been reported, complications such as proximal clavicle fractures, widening of the hook hole, rotator cuff tear, subacromial impingement, and often acromial fractures are not uncommon with more clinical experience over the last decade.³ In our report, we present a rare case of acute AC joint injury treated with acromial hook plate with progressive osteolysis and acromial fracture after application.

2 Case report

A 76-year-old Caucasian woman had a painful disability due to displaced AC joint dislocation (Rockwood type III) after a fall on her left shoulder. The patient had a history of mild stage 2 renal failure 10 years prior to injury, with no history of fracture or diabetes. She underwent open reduction-internal fixation 2 weeks after injury using a Synthes® clavicular hook plate (4-hole plate with a 12-mm hook depth) (Depuy Synthes, Warsaw, IN). There was no soft tissue damage found during the procedure. Patient was instructed to perform pendulum exercise, limit shoulder range-of-motion (ROM) and avoid carrying weights during the first month of surgery (Fig. 1a and b).

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Fig. 1 (a) AP and (b) scapular Y view showing fixation of AC joint using the hook plate at immediate post-operative follow-up. Radiograph findings were unremarkable.

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3 Clinical findings

During the first and second week of post-operative follow-up visits, there were no signs of surgical site infection or tenderness and blood investigations did not yield any abnormalities. However, at both 3 weeks’ and 7 weeks’ follow-up, the patient complained of pain in her AC joint with restricted ROM and limited strength, even though there was no history of new injury to her AC joint.

4 Diagnostic assessment

At 3 weeks’ follow-up, anteroposterior (AP) and scapular shoulder radiograph images confirmed the presence of focal osteopenia and early signs of osteolysis in her left acromial joint (Fig. 2a and b). At 7 weeks’ follow-up shoulder radiograph images showed acromial fracture and osteolysis over the acromial undersurface, distal clavicle and AC joint, and the hook cutting out of the acromion (Fig. 3a and b).

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Fig. 2 (a) AP and (b) scapular Y view at 3 weeks’ follow-up. The acromion shows signs of osteolysis and focal osteopenia.

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Fig. 3 (a) AP and (b) scapular Y view at 7 weeks’ follow-up. There is cutting out of the hook plate implant and fracture of the acromion with slight displacement of AC joint.

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5 Therapeutic intervention

The hook plate was removed and the acromion was secured with Ethibond® sutures. During surgery the coracoclavicular ligaments had scarred and there was minimal superior subluxation of the AC joint. Following removal of the hook plate, radiograph images showed significant displacement of the AC joint together with the acromial fracture (Fig. 4). After revision surgery, patient was instructed to perform pendulum exercise and restrict shoulder ROM, with no weight-related activity.

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Fig. 4 AP view of left shoulder post-removal of the hook plate and TightRope® fixation. There is a fracture of acromion and displaced AC joint distance.

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6 Follow-up and outcomes

Throughout subsequent post-operative follow-ups, there was persistent mild pain but no signs of surgical site complications. Patient underwent a Bone Mineral Densitometry test and there was no evidence of osteoporosis. At 6 months’ follow-up, the patient had mild pain, moderate stiffness, limited ROM on the left shoulder (forward flexion 123^o, abduction 103^o) and reduced strength (7/10 lbs). On CT scan examination we found that the bone was not healed and there was displacement of AC Joint (Fig. 5a). Further revision surgery and bone grafting was offered but the patient declined. Therefore, conservative arm sling fixation was applied to the patient. By 9 months’ follow-up, obvious non-union acromial fracture and AC joint displacement was noted on radiographic examination (Fig. 5b and c). However, the patient had no complaints of pain and was coping well with activities of daily living. Furthermore, ROM and strength did not worsen (forward flexion 123⁰, abduction 103⁰, and strength 7/10 lb).

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Fig. 5 At 9 months’ follow up, post-removal of the hook plate. (a) Sagittal view on CT scan shows non-union at the tip of the acromion bone. (b) AP and (c) scapular Y view of the left shoulder shows a non-progressive displacement of AC joint.

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7 Discussion

In cases of Rockwood Type III AC joint injury, a more aggressive therapy for surgical approach is often needed, although the evidence still remains unclear.^4,5 Currently, management options include hook plate fixation, Bosworth screws, Mersilene® tape (coracoclavicular sling), tension wire fixation, Weaver-Dunn procedure and more recently arthroscopic techniques such as TightRope® fixation.⁴ Over the past two decades, the use of hook plate fixation has been increasingly described. Compared to traditional coracoclavicular fixation methods such as Bosworth screws, hook plate fixation offers better rotational mobility and improved natural healing of the coracoclavicular ligament,^5,6 while providing stable acromioclavicular fixation in displaced or severe AC joint dislocations. However, despite evidence of improved functional outcomes and clinical scores following hook plate fixation,^7,8 complications such as subacromial shoulder impingement and rotator cuff lesions remain common.^9,10 Studies have postulated that this is due to movement of the hook in the subacromial space during shoulder movement, influenced by factors such as hook body-acromion distance and hook body depth.^2,10

In our case report, we describe an arguably more devastating complication in the subacromial space, which is that of acromial osteolysis and fracture with cut-out of the hook plate. Several case studies and series have reported occurrences of symptomatic acromial erosion caused by the hook plate, with majority of cases requiring hook plate removal before the patient eventually regained pain-free and full ROM.^11–13 In our case we found that in 3 weeks follow-up there were already signs of focal osteopenia and bone erosions with only mild clinical manifestation of pain. Biomechanically, this is likely caused by the direct pinpoint pressure of the hook plate and impingement at the undersurface of the acromion, which leads to stress-riser effect with eventual osteolysis erosion and, in worse cases, acromial fractures.^2,11,12,14

For our patient, we detected radiographic evidence of acromial fracture and cut-out at 7 weeks follow-up, accompanied by significant pain and limited ROM. This complication is rare in existing literature, especially in otherwise healthy patients without comorbidities and lower Rockwood Type dislocations. Kang et al. and Chiang et al. both reported cases of younger patients with acromial fracture at 10 weeks and 8 months respectively on a background of Rockwood Type V AC joint dislocation.^3,15 A case-control study by Hamid et al. reported an incidence of hook cut-out, tip fracture and acromial erosion each.¹⁶ We postulate that a possible contributing factor for early acromial fracture in our patient is that of older age, despite the lack of osteoporosis. Our patient had findings of early osteopenia and thus it is important to note the patient’s comorbidities or medications as potential risk factors. In terms of timing, our patient’s hook plate was removed at 7 weeks, which fell within the recommended duration of 8–12 weeks.^11,17 Despite the non-union acromial fracture at 6 months follow-up, we opted for conservative management given the patient’s refusal for surgery, stable function and minimal pain, which is concordant with therapeutic strategies in the literature.^3,15

Our case of acromial fracture with hook plate fixation has raised the importance of considering alternative therapies. Older methods such as Weaver-Dunn have fallen out of favour to more anatomic reconstructions¹⁸ while evidence has found no difference between functional outcomes of Mersilene® tape and hook plate fixation.¹⁹ However, in recent years, with the advent of arthroscopic techniques, TightRope® fixation has become the mainstay of treatment. Hamid et al. showed significantly better 1-year post-operative functional and radiological outcomes in TightRope® compared to hook plate fixation.¹⁶ Interestingly, a recent retrospective analysis by Shen et al. demonstrated no significant difference in longer-term functional outcomes or pain between the two groups, although there was a case of redislocation and acromial osteolysis in the TightRope® and hook plate groups respectively.²⁰

This case report of acromial fracture in an elderly female with hook plate fixation for AC joint dislocation has highlighted a few important points. Firstly, it is important to stress that acromial fracture due to hook plates is an extremely rare complication and has only been described in previous case reports. Secondly, the anatomy of the acromion is too diverse to accommodate a single hook plate. In certain cases, such as our elderly female patient, the tip of the plate may require bending and smaller depths of the hook should be selected if necessary. Thirdly, acromial fracture can be prevented by careful and frequent radiographic examination during the initial 3 months of fracture healing, especially if there are comorbidities or a medication history that may lead to osteopenia. Lastly, stable acromial fractures can be treated conservatively if patient does not have pain and function remains adequate. However, if the fracture is unstable, the patient will likely require an open reduction-internal fixation with a pre-contoured plate and bone graft.

8 Conclusion

This case report is important to the existing literature due to the rarity of acromial fractures and hook plate cut-out. While the use of hook plates for acute AC joint reconstruction yields good reduction and allows early post-operative ROM, this report has demonstrated the importance of considering the risk of fracture and osteolysis in an elderly patient. With the advent of TightRope®, it will be of great clinical significance to have future studies compare the outcomes of hook plate fixation versus TightRope® in an elderly population and those predisposed to osteopenia, based on medication or comorbidities.

Financial support and sponsorship

None.

Informed consent

N/A.

Authors contribution

Conceptualization and Design: M.K·W, D.L, E.W, M.Y.

Acquisition of Data: N/A.

Analysis and Interpretation of Data: N/A.

Writing – original draft: M.K·W, D.L, E.W, M.Y.

Writing – review & editing: M.K·W, D.L, E.W, M.Y.

Disclosures

•Conflict of interest: None•Funding: None•Ethical approval: N/A•Informed consent: N/A