Radiographical magnification of the shoulder region

Reverse total shoulder arthroplasty (RTSA) represents an efficacious surgical intervention for the alleviation of incapacitating shoulder pain. Initially introduced for the treatment of rotator cuff insufficiency, it has demonstrated success in managing various conditions such as glenohumeral osteoarthritis, proximal humerus fractures and malunions, inflammatory arthritis, tumor reconstruction, and irreparable rotator cuff tears [1, 2]. With the aging population, the demand for RTSAs is increasing, and projections indicate a continued rise in the coming decade [3]. Nonetheless, the implantation of RTSA remains a challenging procedure, even for experienced shoulder surgeons [2].

Preoperative planning plays a crucial role in achieving optimal implant size selection and accurate placement of the glenosphere and humeral component [4]. Conventional preoperative planning methods typically involve the utilization of either physical or digital templates on radiographs [5]. These templates need to be calibrated to accurately correspond to the actual size of the shoulder bones. However, in the case of RTSA, both digital and physical radiographic templates commonly employ a fixed 5% magnification factor [6].

Previous studies have demonstrated significant variability in image magnification among different patients. For instance, in the context of hip imaging, magnification levels have been reported to range from 12 to 35% [7, 8]. Similarly, investigations involving trauma patients undergoing humeral nailing or plating have indicated a magnification range of 2–22% for the humerus [9]. It is important to note that radiographic magnification is influenced by various factors, including radiographic techniques and the size of the patient, which can affect both the focus–film distance and the shoulder–film distance [10, 11].

Consequently, certain hospitals employ external markers to estimate magnification and enhance the precision of preoperative templating. Nevertheless, in cases where such markers are unavailable, a fixed magnification value is universally applied to all patients. We hypothesize that (1) the actual magnification significantly deviates from the commonly adopted value of 5%, and (2) notable variations in radiographic magnification exist among patients. To address these hypotheses, we conducted a retrospective observational study wherein the magnification of the shoulder joint was ascertained by employing the implant as an internal calibration marker.

A retrospective study was conducted at Motol University Hospital, Czechia, involving patients who had previously undergone total glenoid arthroplasty. The analysis focused exclusively on the SMR Reverse Shoulder System implants (Lima Corporate, San Daniele del Friuli, Italy; [6]). The study inclusion criteria encompassed the following parameters: unilateral RTSA, documented information regarding implant size and type, recorded patient height and weight, availability of digital anteroposterior (AP) radiographs of the shoulder in a neutral position from the hospital archives, and clear visibility of the humeral and glenoid components of the RTSA. A total of 98 digital AP radiographs from a consecutive cohort of 98 patients, taken during the first follow-up after glenoid arthroplasty, were obtained as DICOM files. Three patients were excluded from the study due to evident arm rotation in the radiographic images, and one patient was excluded due to missing information on the radiographic setup in the DICOM file. The final cohort comprised 94 patients, including 62 female and 32 male patients. The average age of patients at the time of surgery was 69.4 years (±8.7 years, ranging from 38 to 85 years). The data were collected during the period 2014–2017, with the actual study being conducted in April 2023. This noninterventional retrospective study based on an anonymized dataset was approved by the ethics committee of Motol University Hospital (reference no. EK-1204/18). The authors did not have access to any information that could potentially identify individual participants either during or after the process of data collection.

For reference, the diameter of the proximal part of the reverse humeral body (component no. 1352.20.010) was used (Fig. 1). This component possesses a cylindrical shape, and its diameter remains consistent regardless of internal or external rotation. The cylindrical geometry was confirmed by fitting a cylinder to a 3D scan of a nonimplanted specimen using an optical coordinate measuring system (Omnilux, RedLux Ltd., Romsey, UK). The physical diameter of 36.6 mm was obtained from the cylindrical fit and verified by measuring the component using a digital caliper (Mahr GmbH, Göttingen, Germany). The component dimension on the radiographs was estimated from the DICOM files using the Fiji platform for biological-image analysis [12]. Specifically, two points on each side of the cylindrical portion of the component were defined and used to construct the lateral and medial edges of the component. A custom MATLAB script (MATLAB R2020b, The MathWorks, Inc., Natick, MA, USA) was developed to calculate the diameter of the component as the mean perpendicular distance between the lines measured at the defined points (Fig. 1). A single observer (A. K.) analyzed all radiographs. To assess the reliability of the method for estimating radiographic magnification, five independent and blinded observers (postgraduate students of biomechanics at CTU in Prague) analyzed a set of 20 randomly selected radiographs.

The radiographic magnification (M) of the implants was calculated aswhere 0% magnification correspond to exact match between the image size and implant size.

There was an excellent agreement between the observers in evaluating the magnification of the radiographs (inter-rater ICC = 0.997, 95% confidence interval: 0.991–0.999). The average magnification factor was 11.9% (standard deviation [SD]: 3.2%, range: 5.7–20.3%). The magnification factor was normally distributed (Shapiro–Wilk normality test p = 0.209; Fig. 2).

A slightly higher radiographic magnification was observed in male (mean: 12.7%, SD: 3.5%) than in female patients (mean: 11.4%, SD: 3.1%), the difference was not significant (Welch two-sample t test p = 0.077). A linear model was fitted to predict radiographic magnification with patient height and weight. The model’s explanatory power was weak (R = 0.09), indicating large inter-individual variability among patients (Fig. 3). The effect of weight was statistically significant and positive (p = 0.017), while the effect of height was not statistically significant (p = 0.648).

Previous studies have established a correlation between radiographic magnification in preoperative planning for hip arthroplasty and patient factors such as sex, age, weight, or body mass index [15, 16]. In our study, the only statistically significant factor was patient weight, as higher weight correlated with larger radiographic magnification (Fig. 3). The slightly higher magnification observed in male patients (Fig. 2) can be attributed to the observation of higher weight in male compared to female patients (Fig. 3). However, due to the significant inter-individual differences, this correlation (Fig. 3) is not applicable for clinically practical correction.

A limitation of this study is its reliance primarily on data obtained from a single clinical workplace and a single X‑ray machine, which limited the cohort size. Extending the study to multiple clinical workplaces would provide a lager cohort, but it could introduce additional variability related to the specific radiographic setup. Hornova et al. in 2017 [8] demonstrated considerable variations in hip radiographic magnification ranging from 16 to 24% between different clinical workplaces. It should also be noted that the radiographic device used in this study performed an intrinsic magnification correction of 5.2% according to DICOM image headers, where the imager pixel spacing of 0.143 mm appeared as a radiographic image with a pixel spacing of 0.136 mm.

The magnification was estimated from the cylindrical component of RTSA as shown in Fig. 1. The distortion of the cylindrical section due to shoulder flexion or extension was corrected by averaging its dimension along the length of the component. However, the magnification obtained at the cylindrical component might differ from the magnification at the glenosphere, as these two components might not be aligned in the sagittal plane.

Three-dimensional planning techniques using computed tomography (CT) imaging have been proposed for enhancing precision in preoperative planning [17, 18]. However, recent investigations have indicated minor discrepancies between 3D planning and traditional 2D methods [4]. Additionally, careful consideration must be given to the potential risks associated with increased radiation exposure, as higher effective doses during CT shoulder examinations may increase the risk of developing cancer [19]. Nevertheless, one advantage of CT scans is the absence of a need for external calibration markers. Moreover, CT scans are typically readily available, and the radiation dosage from modern CT machines continues to decrease.

Plain radiographs serve as the initial assessment method for evaluating the glenohumeral joint [20]. It has been shown that the use of external markers can enhance the accuracy of preoperative templating. A spherical external calibration marker should be positioned at the anticipated joint height above the detector [5].