24 How I Deal with Humeral Bone Loss in Reverse Shoulder Arthroplasty
This chapter details surgical and implant options, as well as our specific technique of using an allograft-prosthesis composite for managing proximal bone loss when performing a reverse total shoulder. Treatment considerations include the use of an endoprosthesis, convertible stems, and allograft-prosthetic composite. Patient outcomes with allograft, endoprostheses, and cemented reverse shoulder arthroplasty can help in making informed decisions. The senior author’s preferred technique for managing proximal humeral bone loss with allograft-prosthesis composite is presented in two detailed cases.
Proximal humerus bone loss can occur with tuberosity resorption after proximal humerus fracture, difficult arthroplasty resection, infection, and tumor excision. With increasing amounts of proximal bone loss, the attachment of the rotator cuff and/or deltoid may be compromised, which results in problems of soft-tissue tensioning, instability, and prosthetic fixation.
24.1.1 Bone Loss
The amount of bone loss dictates choice of treatment. Bone loss can be cavitary where there is loss of cancellous bone, but the cortex is preserved. However, in some cases, both cancellous and cortical bone is lost.
In cases where there is cavitary loss without cortical compromise, morcellized cancellous bone can be packed into the defects after prosthetic insertion. When there is loss of the greater and lesser tuberosities but the deltoid is functionally intact, a reverse prosthesis may be useful. If the bone loss results in functional insufficiency of the deltoid, a reverse arthroplasty may not be possible. In that situation, an endoprosthesis may be more appropriate.
24.2 Options for Reconstruction
24.2.1 Humeral Prosthetic Design Considerations
Monoblock versus Modular Humeral Component
A monoblock prosthesis is when the entire implant is a single unit (mono = one). The monoblock prosthesis can be hemiarthroplasty or can be the humeral side of a reverse shoulder arthroplasty. In the case of a hemiarthroplasty, it has a fixed head-neck angle, head size, and offset. A modular component allows for greater implant customization to accommodate each patient’s anatomy. In the case of a reverse shoulder arthroplasty, these modular humeral components have a risk of failure at the interface between the humeral tray and stem. Micromotion of the humeral tray can cause it to separate from the stem, leading to failure of the prosthesis.1 A biomechanical evaluation of torsional stability of monoblock and modular prostheses found that with bone loss a monoblock prosthesis failed less frequently than a modular prosthesis.2
Conventional versus Reverse Arthroplasty
Conventional (or anatomic) shoulder arthroplasty is an appropriate implant choice in the setting of arthritis (primary or posttraumatic) in patients with an intact and functioning rotator cuff. The rotator cuff function is essential to keep the humeral head centered allowing for normal joint mechanics and centered wear on the glenoid. In a reverse shoulder arthroplasty, the glenoid is replaced with a glenosphere and the humeral head is replaced with the socket. This changes the joint mechanics, allowing the deltoid to compensate for a torn rotator cuff. The stability of the reverse shoulder arthroplasty is primarily from compressive forces from the soft tissue, mainly the deltoid. The depth of the humeral socket and to a lesser extent implant size play a role in prosthetic joint stability.3
Companies are now developing modular stems that can be used in a standard hemiarthroplasty, a total shoulder arthroplasty, or a reverse shoulder arthroplasty. This type of stem allows conversion from an anatomic or hemiarthroplasty to a reverse arthroplasty without removing the stem. The prosthetic head and neck are removed and the humeral tray is attached. Research has showed significant decrease in surgical time, blood loss, and complications with a convertible stem compared to revising the humeral component.4,5,6
Many prosthesis companies have also developed fracture stems to be used when performing a reverse arthroplasty in the setting of a three- or four-part proximal humerus fracture (► Table 24.1). The design rationale of a fracture stem is to improve tuberosity healing. Some fracture stems have a window for a bone graft which may assist healing.7 Many fracture components also have holes in the stem to pass the rotator cuff sutures through, assisting in the reduction of the tuberosities.
Custom stems may be necessary when the humerus is deformed due to congenital defects or the sequelae of a malunited fracture.
While endoprostheses have been frequently used for proximal humeral reconstruction after tumor resection, their use with complex humerus fractures and severe bone loss is limited to a few case reports.8 In the tumor literature, endoprosthesis refers to a hemiarthroplasty that also replaces the tuberosities as well as a varying amount of humeral diaphysis, depending on the amount of bone loss. (There are studies using a reverse arthroplasty endoprosthesis implant, but that implant is not currently available in the United States.)
With an endoprosthesis, shoulder instability is a critical issue since in many cases of tumor resection the rotator cuff and axillary nerve must be sacrificed. Researchers have attempted a number of techniques to stabilize the endoprostheses, but no single technique has been widely adopted. When the axillary nerve and deltoid function is preserved, stability increases, but shoulder function commonly remains suboptimal.9 Cannon et al measured range of motion after endoprosthesis reconstruction for tumor resection with mean abduction of 41 degrees (range: 10–90 degrees) and mean active forward flexion of 43 degrees (range: 5–115 degrees).10
24.2.2 Bone Graft Options Allograft
Allograft augmentation to help restore bone loss is an option when coupled with reverse shoulder arthroplasty. Allograft may improve bone stock for future procedures, provide a reattachment site for the rotator cuff muscles, improve contour of the shoulder, restore more anatomic muscle tension, and increase rotational stability at the prosthesis–bone interface.2,11,12,13 The disadvantages of using an allograft include risk of nonunion/malunion, increased operative time and complexity, cost of allograft, infection, and disease transmission.
There are a number of different ways to prepare the allograft, which is usually a fresh-frozen allograft consisting of the entire proximal humerus including head, neck, tuberosities, and soft-tissue attachments. One technique using a fresh-frozen allograft is to remove the cancellous bone after making the anatomic neck cut. Then, a step cut is made with 5 cm of bone laterally and 1 to 2 cm of bone medially. A corresponding step cut is made on the patient’s humerus.12,13 Another method is to create a chevron osteotomy or a straight transverse osteotomy.14,15
To attach the allograft to the humeral shaft, heavy nonabsorbable suture, cerclage cables, or a plate with unicortical fixation have been described.14 The plate is usually placed along the anterolateral aspect of the humerus. A combination of cerclage wires and unicortical screws can be used to affix the plate.16
Vascularized Fibular Reconstruction
In cases where there is massive bone loss and there is not enough distal bone to support a prosthetic stem, vascularized fibular reconstruction is an option. In general, vascularized bone theoretically has improved capacity to heal. In addition, the bone has the capacity to remodel. Once the fibula has incorporated, the bone may be used to support a prosthesis.