4. Discussion
fracture, fractures
ADVERSE EVENTS, STILL
The Xin1 implant positioning system, as introduced in [Anatomical locking plates such as the PHILOS system stabilize proximal humerus fractures by spanning a multitude of screws in the often-osteoporotic head segment. It is aimed to anchor the screw tips in the subchondral bone region, close to the cartilage layer. The task is surgically demanding due to an eminent risk of screw penetration into the joint space while trying to judge an oblique screw path inside a spherical anatomical structure from planar X-ray projections. On the other hand, too-short screws miss anchorage [In the scope of this work, the prototype Xin1 system was evaluated in terms of performance in both postmortem and clinical settings. The system estimates the plate position and calculates all required screw lengths to reach into the subchondral bone area from a pair of conventional C-arm images. Xin1 outperformed the conventional freehand technique in terms of surgical precision (52% error reduction), procedure time (38% shorter) and radiation exposure of OR personnel and the patient (64% less X-rays). Particularly for medical professionals, the continuous exposure to ionizing radiation during surgery is a frequently addressed problem [The postmortem experiment was performed on full-body specimens with compromised soft tissue mantle due to embalmment and previous use. However, we believe that the experiment carried high relevance for judging the technology before entering into clinical testing. One methodological issue, though, could not be neglected, which was the absence of bone fractures. In a second phase, we hence conducted a clinical registry on fracture patients to investigate the potential effect of bone fragment shifts on the system output, and at the same time to analyze the clinical handling of the Xin1 marker clip. Safe handling was shown in all 11 included patients by the absence of device-related adverse events. To minimize risks to the patient at the current state of system development by potentially falsified system outputs, X-ray image analysis was performed in a retrospective fashion without providing system information to the operator during surgery. The missing feedback in the operating theatre reduced risks but also led to a high number of patients, who needed to be excluded from the image-based evaluation (five out of eleven). These dropouts are deemed to be artifacts from the retrospective study design and may be avoided in the future when real-time feedback is provided to the operator. A stereotactic system always relies on an angular spread of at least two view ports to reconstruct a 3D situation [Besides these general observations, the clinical study confirmed the feasibility of the marker clip to be embedded into hospital and surgical workflows. The system software demonstrated robust performance in presence of real-life bone fractures. Certain head fragment shifts due to manipulations during the screw insertion were suggested by differences in the computed screw lengths and measured plate position before and after screw insertion. Shifts were generally small, but in one case they reached 10 mm in the dorsal–ventral direction. Clinical consequences such as screw perforation into the joint cannot be excluded in such cases. It is therefore proposed to perform a verification measurement with the Xin1 system after the head fragment has been fixed in place with the first screw. On a critical note, it cannot be ruled out that the measured differences are artifacts related to inaccuracies of the system rather than describing actual bone fragment shifts. In this regard, the segmentation procedure to determine the position of the humeral head, still relying on user input, might play an important role. Inaccurate determination of the humeral head boundaries will impact the accuracy of the system. Fully automated segmentation may be desired in the future. In a previous bench experiment [Study limitations are summarized as the following: The postmortem experiment was performed on unfractured embalmed body donors with compromised soft tissues. The effect of intact soft tissues and fractured proximal humerus was hence difficult to estimate from this sub-study. Furthermore, the tip–joint distance evaluation assumed a constant cartilage layer thickness, which naturally varies between locations and individuals. Procedure time assessment did not take system setup and takedown durations into account, and thus represents only a net value. The clinical registry allows only for preliminary conclusions at this early development stage because of the missing system feedback in the operating room. This also led to a considerable number of dropouts lowering the sample size for image evaluation. As desired for a clinical handling investigation, several surgeons shared the surgeries. This, on the one hand, gave valuable insights about marker clip usage in different hands, but might have contributed to the large number of excluded datapoints for image analyses. However, several confounders regarding the Xin1 approach for proximal humerus plating could be identified from the studies, including: potential marker movements, risk for use of wrong implants and instruments, omitted use of a screw guide, bone fragment shifts due to surgical manipulation, a too-narrow X-ray view angle or the segmentation procedure of the humeral head requiring manual interaction. Furthermore, some technical limitations rendering validation and regulation of such assistance devices difficult are to be mentioned. Optical image distortion of older image intensifiers can drastically influence the system accuracy. Flat panel-based C-arms solve this issue and currently replace image intensifiers around the globe. Still, interfacing to the imaging modalities remains an issue. A variety of analog and digital output protocols make connecting external devices difficult. Important parameters such as flip or rotation flags are not transmitted. The ability to run third party image processing software directly on the C-arm would be an important prospect for future developments, dramatically increasing usability and effectiveness of assistance technologies such as Xin1.