Opinion editorial—reconstructing the glenoid in total shoulder arthroplasty

ABOUT THE EXPERT

Joseph D. Zuckerman, MD, is the Walter A.L. Thompson Professor of Orthopedic Surgery at the New York University Grossman School of Medicine and Chair of the Department of Orthopedic Surgery. He has served as President of the American Shoulder and Elbow Surgeons and the American Academy of Orthopedic Surgeons. Dr. Zuckerman’s clinical and research interests have focused on shoulder arthroplasty and the development of implants and techniques for performing the procedure.

Total shoulder arthroplasty (TSA) is recognized as a reliable and reproducible procedure to treat a wide range of glenohumeral pathology. The development and broader use of reverse total shoulder arthroplasty (RTSA) have only expanded the indications for TSA and enhanced our ability to address failed anatomic total shoulder arthroplasty (ATSA). The increasing number of shoulder arthroplasties performed each year is evidence of these trends.

Every aspect of TSA—preoperative evaluation, surgical approach, humeral implant, glenoid implant, soft tissue management, and postoperative rehabilitation and outcomes—has been and continues to be studied extensively. Even in that context, the glenoid has received more attention than any other aspect of the procedure, which makes it reasonable to ask, “why?” There are a few reasons. First, the implantation of the glenoid is considered one of the more challenging aspects of the procedure because of exposure, size of the glenoid, and the associated deformity. Second, there has been longstanding concern about the survival of the glenoid component because of the incidence of radiolucent lines and the potential relationship to glenoid component loosening. Third, our ability to define a successful reconstruction remains challenging because we do not have sufficient long-term studies that correlate with implant position, size, type, and fixation. And fourth, although most still consider the glenoid the weakest link of the procedure in terms of implant survival, this may not be the case. Glenoid component loosening may be related more to gradual rotator cuff dysfunction over time and the eccentric loading that results.

Replacing the glenoid is an essential component of TSA for both anatomic and reverse, and in that context, our ability to reconstruct the glenoid—to correct deformity that occurs in a variety of glenohumeral disorders—is essential to the medium- and long-term survival of the implant. My experience over the last 30+ years has led to some conclusions about glenoid reconstruction that I would like to share.

Glenoid anatomy

The anatomy of the glenoid is quite variable ( Fig. 63.1 ). Although most glenoids are described as pear-shaped, a significant number are oval. ^, Anatomic studies have documented the glenoid width to be approximately 27 mm with values ranging from 20 to 35 mm and an average difference of 4.2 mm between men and women. Average glenoid height is 38 mm with values ranging from 29 mm to 50 mm, with an average difference of 4.7 mm between men and women. ^, Because of the shape of the glenoid, the anterior-posterior dimension of the upper half is generally smaller than the lower half with a ratio of 0.8 to 1.0. Glenoid version is also variable, and studies have documented normal measurements as averaging between 2 degrees of anteversion and 8 degrees of retroversion. Glenoid inclination also shows considerable variability ranging from 12 degrees of superior inclination to 11 degrees of inferior tilt. ^, The range of values for version and inclination are important because the variability makes it difficult to determine an exact preferred glenoid component position and glenoid reconstruction for any specific patient.

The anatomy of the glenoid vault is as important as the anatomic characteristics of the glenoid surface ( Fig. 63.2 ). The glenoid vault lies between the articular surface of the glenoid and the body of the scapular and is comprised primarily of cancellous bone with a thin rim of cortical bone. Computed tomography (CT) scans clearly show its triangular shape. It narrows as it progresses from lateral to medial. It is also important to recognize the difference between glenoid “face anatomy” and “vault anatomy” ( Fig. 63.3 ). The glenoid vault is often not located centrally. To achieve placement of the central fixation peg or threaded post in the deepest position, the vault anatomy and location in relation to the glenoid face must be clearly understood. Surgical technique guides often describe drawing the x -axis and y -axis in the glenoid face and using the point of intersection as the central point for glenoid placement ( Fig. 63.4 ). Variations in glenoid anatomy, especially in the context of erosion, make this technique unreliable. The glenoid vault is very important because it serves as the primary bony support for the glenoid component. The less bone available in the vault as a result of glenoid erosion, the more difficult the glenoid reconstruction becomes. If the glenoid component is placed eccentrically or if fixation pegs perforate the vault, implant longevity may be compromised. For these reasons and others, it is clear that the glenoid is the most valuable “real estate” in the shoulder.

Glenoid classifications

An appreciation of normal glenoid anatomy and its variations is essential to understand the pathoanatomic changes that occur in a variety of degenerative conditions. Different classification systems are utilized to describe the changes in glenoid pathology. The Walch Classification is the most commonly utilized and focuses on the degree of posterior and central glenoid erosion and, to a lesser extent, anterior erosion. The Favard Classification focuses on glenoid erosion in the superior-inferior plane and is most commonly utilized in cases of rotator cuff arthropathy. This classification focuses more on glenoid bony changes than the classifications previously described by Hamada and Seebauer.

These classifications are only useful if we have a clear understanding of the bony anatomy. Understanding glenoid anatomy requires appropriate imaging studies. For the past 10 years, I have obtained standard radiographs and a CT scan with three-dimensional reconstructions on every patient undergoing TSA in order to have as comprehensive an understanding of the glenoid pathoanatomy as possible ( Fig. 63.5 ). Prior to that time, I was much more selective in which patients I obtained a preoperative CT scan. I relied more heavily on the axillary view thinking that I could assess glenoid deformity adequately. However, the two-dimensional representation obtained on the axillary view provides much less reliable information than a CT scan, particularly at present when complex three-dimensional reconstructions are available (see Fig. 63.5 ). It is now quite clear to me that a CT scan is absolutely essential for every patient. It allows me to better understand the glenoid anatomy and to plan the surgery. In addition, the development and utilization of preoperative planning software and particularly intraoperative navigation has made preoperative CT scan mandatory. Regardless of the preoperative imaging protocol utilized, one basic principle applies: any glenoid reconstruction or replacement must address the underlying bony changes, which cannot be accomplished unless the operating surgeon has a clear understanding of the pattern and degree of changes present.