Patient Posture Affects Simulated ROM in Reverse Total Shoulder Arthroplasty: A Modeling Study Using Preoperative Planning Software

Abstract

Background: Component selection and placement in reverse total shoulder arthroplasty (RTSA) is still being debated. Recently, scapulothoracic orientation and posture have emerged as relevant factors when planning an RTSA. However, the degree to which those parameters may influence ROM and whether modifiable elements of implant configuration may be helpful in improving ROM among patients with different postures have not been thoroughly studied, and modeling them may be instructive.

Questions/purposes: Using a dedicated expansion of a conventional preoperative planning software, we asked: (1) How is patient posture likely to influence simulated ROM after virtual RTSA implantation? (2) Do changes in implant configuration, such as humeral component inclination and retrotorsion, or glenoid component size and centricity improve the simulated ROM after virtual RTSA implantation in patients with different posture types?

Methods: In a computer laboratory study, available whole-torso CT scans of 30 patients (20 males and 10 females with a mean age of 65 ± 17 years) were analyzed to determine the posture type (Type A, upright posture, retracted scapulae; Type B, intermediate; Type C, kyphotic posture with protracted scapulae) based on the measured scapula internal rotation as previously described. The measurement of scapular internal rotation, which defines these posture types, was found to have a high intraclass correlation coefficient (0.87) in a previous study, suggesting reliability of the employed classification. Three shoulder surgeons each independently virtually implanted a short, curved, metaphyseal impaction stem RTSA in each patient using three-dimensional (3D) preoperative surgical planning software. Modifications based on the original component positioning were automatically generated, including different humeral component retrotorsion (0°, 20°, and 40° of anatomic and scapular internal rotation) and neck-shaft angle (135°, 145°, and 155°) as well as glenoid component configuration (36-mm concentric, 36-mm eccentric, and 42-mm concentric), resulting in 3720 different RTSA configurations. For each configuration, the maximum potential ROM in different planes was determined by the software, and the effect of different posture types was analyzed by comparing subgroups.

Results: Irrespective of the RTSA implant configuration, the posture types had a strong effect on the calculated ROM in all planes of motion, except for flexion. In particular, simulated ROM in patients with Type C compared with Type A posture demonstrated inferior adduction (median 5° [interquartile range -7° to 20°] versus 15° [IQR 7° to 22°]; p < 0.01), abduction (63° [IQR 48° to 78°] versus 72° [IQR 63° to 82°]; p < 0.01), extension (4° [IQR -8° to 12°] versus 19° [IQR 8° to 27°]; p < 0.01), and external rotation (7° [IQR -5° to 22°] versus 28° [IQR 13° to 39°]; p < 0.01). Lower retrotorsion and a higher neck-shaft angle of the humeral component as well as a small concentric glenosphere resulted in worse overall ROM in patients with Type C posture, with severe restriction of motion in adduction, extension, and external rotation to below 0°.

Conclusion: Different posture types affect the ROM after simulated RTSA implantation, regardless of implant configuration. An individualized choice of component configuration based on scapulothoracic orientation seems to attenuate the negative effects of posture Type B and C. Future studies on ROM after RTSA should consider patient posture and scapulothoracic orientation.

Clinical relevance: In patients with Type C posture, higher retrotorsion, a lower neck-shaft angle, and a larger or inferior eccentric glenosphere seem to be advantageous.

Fig. 1

A-B (A) Illustrations and (B) three-dimensional CT images of patients with Types A, B, and C posture show increasing scapular internal rotation, anterior tilt, protraction, and drooping as well as kyphosis and a barrel-shaped chest according to the current study’s results and a previous study’s results [19].

Fig. 2

Overview of study methods. Whole-torso CT scans were loaded into the modified RTSA planning system, using body axes as a coordinate system. Patients were grouped into three different posture types based on their scapulothoracic orientation (Types A, B, and C), and their respective simulated ROM after virtual RTSA implantation with different components and configurations was analyzed.

Fig. 3

A-D (A) This figure shows the manual selection of two of the multiple landmarks in the middle of the spine and sternum in a patient with Type C posture to define the sagittal plane of the body, which is used to align the humeral rotation to 0° in the simulation. Scapulothoracic orientation is determined as follows: (B) The scapula’s internal rotation is the angle between the scapula’s transverse axis (red line) projected onto the transverse plane and the transverse axis (red arrow). (C) The scapula’s upward rotation is the angle between the scapula’s transverse axis (red line) projected onto the coronal plane and the transverse axis (red arrow). (D) The scapular tilt is the angle between an orthogonal axis to the scapular plane (red line) projected onto the sagittal plane and anterior axis (green arrow).

Fig. 4

This illustration shows the different changes in humeral component alignment to the proximal humeral metaphysis evaluated in this study, including torsion and inclination. Torsion is rotation of the component around its own axis, and inclination and version are tilt of the component tray or cup in relation to the component’s shaft axis, 90° perpendicular to each other.

Fig. 5

This graph shows the median simulated ROM and interquartile range in different planes of motion depending on the posture type, irrespective of the component configuration in RTSA. Significant differences are marked by ^ap < 0.01.

Fig. 6

A-D These graphs show the median simulated ROM and interquartile range in different planes of motion depending on the humeral component’s retrotorsion, analyzed by (A) Type A posture, (B) Type B posture, and (C) Type C posture. (D) This graph shows the combined motion score (points = the sum of all median ROM values) for each posture type depending on the humeral component’s retrotorsion. Significant differences are marked by ^ap < 0.01. IR = internal rotation.

Fig. 7

A-D These graphs show the median simulated ROM and interquartile range in different planes of motion depending on the humeral component’s inclination, analyzed by (A) Type A posture, (B) Type B posture, and (C) Type C posture. (D) This graph shows the combined motion score (points = sum of all median ROM values) in each posture type depending on the humeral component’s inclination. Significant differences are marked by ^ap < 0.01.

Fig. 8

A-D These graphs show the median simulated ROM and interquartile range in different planes of motion depending on the glenoid component’s configuration, analyzed by (A) Type A posture, (B) Type B posture, and (C) Type C posture. (D) This graph shows the combined motion score (points = sum of all median ROM values) in each posture type depending on the glenoid component’s configuration. Significant differences are marked by ^ap < 0.01.

Fig. 9

A-B These images show a patient with Type C posture and high scapular internal rotation. (A) A virtual implantation of an RTSA implant with the humeral component in 0° of retrotorsion. With the arm in neutral rotation, unbalanced opposition of the humeral and glenoid component can be observed. (B) A virtual implantation of an RTSA implant with the humeral component retrotorsion matching the scapula’s internal rotation. With the arm in neutral rotation, balanced opposition of the components is visible.