It is widely accepted that the TAD is a highly significant risk predictor of mechanical failure due to cut-out. However, the concept of the TAD has some limitations. Firstly, it is not practical as the TAD is not routinely available intra-operatively. However, assessment of the TAD is required in real time in the operating theatre to serve as an indicator for ideal lag screw placement. Davies et al. suggest using the TAD for a targeted approach to follow up by bringing back those patients with a high TAD for follow-up , but the ultimate goal should be to avoid poor lag screw positioning in the first place and to evolve from a retrospective assessment method to an intraoperative quality tool. Available computerized navigation systems improve the accuracy of implant placement , but require markers and pre-operative configurations and thus are time-consuming . Atesok and Schemitsch conclude that the proposed advantages of computer-assisted trauma surgery - increased precision, less radiation, and minimised invasiveness - come at the price of major disadvantages, including increased surgical time, a considerable learning curve, cost, as well as special requirements with regards to equipment handling and operating room settings . The ADAPT system provides both TAD and TSD intraoperatively. Our results show that its use does not lead to an increase in surgical time, while accuracy is improved and the radiation exposure is decreased. These effects are achieved with little modification of current surgical and image intensification equipment.
Secondly, the measurement of the TAD can be challenging, especially for inexperienced surgeons. If calculated manually, it is prone to errors and not exact; inter-observer variability was shown to range around 10% [20, 34]. Modern picture archiving and communication systems (PACS) meet the requirements for accurate and reproducible measurement of the TAD , but again are not practical. The presented system features an automatic and objective calculation of the essential values that serve as a strong predictor of lag screw cut-out in real time, independent of the surgeon’s level of experience. Our data shows that the use of the ADAPT system offers reproducible results.
Thirdly, the TAD concept contains a weakness in focusing on distance and neglecting direction; a recent study found only its AP part to be predictive for failure of fixation . Still, there is no clear consensus about the ideal position of the lag screw in the caudal-cranial direction. A recent biomechanical analysis found an inferior lag screw placement to feature the highest axial and torsional stiffness . De Bruijn et al. recently supported this result with their retrospective study from 2012 on the reliability of predictors for cut-out by identifying the central-inferior and anterior-inferior positions as being highly protective against lag screw cut-out . In another recent study from 2011, Herman et al. defined a “safe zone” for the placement of the lag screw . Implantation of the lag screw outside this zone was shown to be thirteen times more risky in terms of mechanical failure (Odds Ratio 13.4). Remarkably, this “safe zone” was within the inferior half of the femoral head. However, a peripheral lag screw position inherently increases the TAD as the distance to the apex of the femoral head grows . Thus, the explanatory power of the TAD concept diminishes with an eccentric lag screw placement. As shown in the excursus, the minimisation of TAD based on 2D fluoroscopic images during lag screw placement can in extreme cases even lead to articular surface penetration. In contrast, the TSD is a meaningful measure regardless of the relative position of the lag screw within the femoral head. Because it computes the real 3D distance of the tip of the lag screw to the surface of the femoral head, the presented system supports the insertion of the lag screw in all surgical cases, including eccentric placement of the lag screw. Hence, the concept of TSD seems critical for surgeons who choose to place the lag screw in an inferior or non-centre-centre position.
In their study on the characteristics of 57 cut-outs with biomechanical explanation as observed in 3066 consecutive patients treated with Gamma Nails, Bojan et al. identified the combination of three critical factors to drive the risk for mechanical failure due to lag screw cut-out: a complex fracture type, non-anatomical reduction and a non-optimal lag screw position . One individual factor or the combination of two did not explain a cut-out. Hence, by avoiding of non-optimal lag screw position as a contributing factor, a significant reduction in cut-out rates may be achieved.
Awareness of the TAD alone has been shown to reduce the rate of mechanical failure due to an improved position of the lag screw; as a result of increased awareness, the quality of reduction was enhanced . The TAD has been confirmed to be a clinically useful indicator for screw placement. This proven concept can be extrapolated to using real-time TSD measurements. The presented novel system is especially useful for less experienced surgeons as the system enables them to achieve TAD and TSD as accurate as the experienced surgeons. Hence, it may be used ideally for learning purposes as the surgeons get direct feedback in real time on both TAD and TSD.
Our results show that both experienced as well as less experienced surgeons can benefit from the ADAPT system. It seems to be particularly powerful in reducing the variability; lag screws that are placed either extremely close to the cortex or extremely far from the cortex involve a particularly high risk of mechanical failure.
A weak point in our study is that the data on fluoro and procedure times were not usable for the experienced surgeons due to a defect of OR equipment (C-arm) in one of the cadaveric tests that influenced measurements. Further studies should concentrate on these end points. Moreover, our findings relate to a cadaveric setting which results in further limitations: the surgeries in the cadaveric tests were performed on unfractured bones. In clinical cases, complex fracture patterns could impact the surgeon’s ability to accurately position the implants. Furthermore, we did not test the impact of the improved accuracy in terms of an optimized TSD on the strength of fixation as our intent was to study the effect of the ADAPT system on the accuracy of implant placement. However, several studies analysed the correlation of the TAD and the likelihood of a cut-out [10, 12, 14, 15, 20–22]. Further biomechanical or clinical studies should be undertaken to investigate whether these findings can be extrapolated to the TSD measurement and whether the TSD proves useful as an intraoperative assessment tool.