Patients and study design
After approval by the institutional ethics committee (Lausanne University Hospital CER-VD, protocol 136/15), we retrospectively reviewed all consecutive cases treated with aTSA in our tertiary referral hospital between January 2002 and December 2014 (n=262). All patients were operated through a deltopectoral approach by the same senior shoulder surgeon for the following clinical indications: primary glenohumeral OA (n=200), post-traumatic glenohumeral OA (n=31), avascular necrosis of the humeral head (n=11), inflammatory arthritis (n=12), or another diagnosis (n=8). The cemented Aequalis all-polyethylene keeled glenoid component (Wright-Tornier, Bloomington, MN, USA) was implanted after minimum glenoid bone reaming to preserve the bone stock. Holes for keeled glenoid implants were drilled using proper instrumentation, and high-viscosity bone cement was vacuum mixed and applied to fix them. All glenoid implants (small, medium, or large) were adapted to the size of the patient’s glenoid cavity according to the manufacturer’s recommendation chart for heads/glenoids diameter mismatch.
Of these 262 cases, we included all patients who had undergone preoperative shoulder CT scans (n=184). The following exclusion criteria were subsequently applied: patients lost to follow-up (<2 years) (n=51), shoulder CT arthrography (n=32), non-arthrographic shoulder CT scans with metal artifacts or incomplete CT coverage of the glenoid (n=3), septic loosening of the glenoid implant (n=3), glenoid implant malpositioning (n=1), and recurrent postoperative scapulohumeral subluxation (n=1). CT arthrograms were excluded because the intra-articular iodinated contrast medium interfered with glenoid BMD measurements due to beam hardening artifacts [23, 24]. The resulting study population consisted of 93 patients, with a mean age at surgery of 69.2 years (range, 45.9‒88.2 years), a female/male ratio of 69/24, a mean body mass index (BMI) of 27.4 (range, 18.4‒42.4), and a smoking history in 10/93 patients (10.8%).
Shoulder CT protocol
All preoperative non-arthrographic shoulder CT scans were performed on 8-, 16-, or 64-detector row CT systems (LightSpeed Ultra, LightSpeed Pro 16, LightSpeed VCT, and Discovery CT750 HD; GE Healthcare, Waukesha, WI, USA) using the following standardized data acquisition settings: tube potential, 120‒140 kVp; tube current, 144‒440 mA; and gantry revolution time, 0.5‒0.8 s. The image reconstruction parameters were as follows: field of view 14×14‒32×32 cm (thus yielding in-plane pixel sizes of 0.27×0.27‒0.63×0.63 mm); section thickness, 0.6‒3.0 mm; section interval, 0.3‒2.0 mm; and sharp (bone or bone plus, GE Healthcare) kernels
CT assessment of preoperative glenoid BMD and morphology
Preoperative glenoid BMD was quantified in 3D from preoperative shoulder CT datasets using the same reliable method described in detail elsewhere [22]. Briefly, we measured the average CT numbers (CTn; in Hounsfield unit, HU) in six contiguous 3-mm-thick VOIs (cortical bone (CO), subchondral cortical plate (SC), subchondral trabecular bone (ST), and three successive adjacent layers of trabecular bone (T1, T2, and T3)) defined within a 40-mm-high cylinder aligned with the mediolateral scapular axis, centered on and adjusted to include the entire glenoid cavity, and whose medial base was positioned at the spinoglenoid notch (Fig. 1). Within this cylinder fully encompassing the glenoid, bone mineral tissue was then segmented using a lower threshold of 300 HU [25]. All these measurements were performed using the Amira software (Thermo Fisher Scientific, Waltham, MA, USA) and user-defined Matlab scripts (MathWorks, Natick, MA, USA).
In addition, preoperative glenoid morphology was assessed on a picture archiving and communication system workstation (Vue PACS; Carestream Health, Rochester, NY, USA) by a board-certified orthopedic surgeon and a senior musculoskeletal radiologist independently, using the updated Walch grading system [26]. In the case of discrepancy, consensus was reached with a senior shoulder surgeon.
Aseptic loosening of glenoid implants
Aseptic loosening of glenoid implants was assessed using conventional shoulder radiographs (anteroposterior and axial/axillary, with/without lateral/scapular “Y” views) performed at regular follow-up visits at 3, 6, 12, and 24 (+/‒1) months, followed by once a year or every two years, depending on the patient’s clinical course. In the case of radiological uncertainty and/or onset of clinical symptoms (e.g. pain, feeling of instability or locking, decreased range of motion), shoulder CT arthrography was performed. Radiographic prosthetic loosening was defined as the presence and/or enlargement over time of complete (thickness >1.5 mm) radiolucent lines at the glenoid bone-cement interface, and/or migration (>5 mm), tilt (>5°), or shift of the glenoid component [2, 5, 8, 27, 28]. The same observers as above independently reviewed all shoulder radiographs and, when available (n=11), postoperative CT arthrograms, with the same consensus agreement approach in the case of discrepancy.
Statistical analysis
We first performed a Kaplan-Meier estimator to evaluate the survival (radiographic aseptic loosening as described above) of glenoid implants in the overall aTSA study population. Survival rate was assessed at 5 and 10 years postoperatively. This analysis was supplemented with univariate Cox proportional hazards regression models to estimate the effect of preoperative glenoid BMD (reflected by CTn in the various predefined VOIs) on glenoid implant survival (defined by the absence of aseptic glenoid loosening). We further analyzed the influence of gender, age, BMI, and smoking history on implant survival.
In a second step, we compared two patient groups. All patients with glenoid implant aseptic loosening were included in the loosening (LSG) group, while the control (CTR) group was formed by matching patients in terms of follow-up time, gender, age, BMI, smoking history, and Walch glenoid types. The follow-up time in the CTR group was at least 2 years. Continuous and categorical variables were compared using one-tailed independent-samples Student’s t-test (or Wilcoxon signed-rank test in the case of skewed distribution with Lilliefors test) and chi-squared test, respectively. We tested the hypothesis that preoperative glenoid BMD (CTn) was numerically lower in the LSG than in the CTR group. Statistical significance was set at p<0.05. The effect size was measured using Cohen’s d standardized difference between the two means, and interpreted as small (d≤0.2), medium (d=0.5), or large (d≥0.8) [29]. Statistical analysis was performed using Matlab’s Statistics and Machine Learning Toolbox (MathWorks) by one of the authors (AT).