Vol.:(0123456789) 1 3 European Journal of Trauma and Emergency Surgery https://doi.org/10.1007/s00068-020-01514-x ORIGINAL ARTICLE A novel lower bound for tip‑apex distance Omer Subasi 1  · Lercan Aslan 2  · Mehmet Demirhan 2  · Aksel Seyahi 2  · Ismail Lazoglu 1 Received: 25 June 2020 / Accepted: 25 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Purpose The cut-out of the cephalomedullary nail is among the most common post-surgery complications for intertro- chanteric fractures. As a risk predictor, a tip-apex distance (TAD) below 25 mm, observed from orthogonal fuoroscopic views, is recommended in the literature. This study aims to demonstrate that TAD < 25 mm is a mathematically insufcient risk defnition and to complement the TAD upper bound with an appropriate lower bound, with the introduction of a novel distance parameter, TADX, based on the orthogonal projection of the nail tip on the central femoral midline. Method Through a mathematical simulation software, all the possible points that lie inside the AP and lateral views of the proximal femoral hemisphere are utilized to create a 3D grid that is sorted into geometrically safe and risk-bearing regions. Extending this methodology, TAD < 25 mm, 10 mm < TAD < 25 mm, and the ideal tip position volumes are simulated. Finally, intersection volumes are created by a combination of diferent candidate lower TADX bounds and TAD < 25 mm upper bound to determine satisfactory TADX limits. Results Simulation of TAD-bound zones exposed that TAD is only a mathematically suitable parameter for defning the upper boundary but not the lower boundary for the optimal region. However, using a TADX lower limit creates a 3D volume that is much closer to the optimal tip region volumetrically and can still be as quickly calculated from 2D AP and lateral views. Conclusions According to the mathematical simulations, the use of a TADX lower bound of 9 mm for small, 7.5 mm for medium, and 7 mm for large femoral heads in conjunction with a TAD upper bound of 25 mm is suggested. Keywords Tip-apex distance · TAD · Intertrochanteric fracture · Proximal nailing · Fluoroscopy Introduction Intertrochanteric femur fractures have a high risk of morbid- ity and mortality [1, 2]. However, such fractures are being successfully treated today via various implants with only 3% of complication risk [3]. The most frequent complication is the cut-out of the nail or the screw through the femoral head that results in penetration of the joint [4]. Cut-out and simi- lar failure modes like cut-in and cut-through signifcantly hamper the necessary mechanical fxation of the implant, causing delayed, malunion, or nonunion of the fracture [5, 21]. Implantation complications could further stem from factors such as the trabecular bone density of the patient, pattern, and severity of the fracture and the success of the anatomical reduction during surgery [6]. The tip-apex distance (TAD), introduced to the litera- ture by Baumgaertner et al. in a 1995 study, is among the most investigated risk parameter for femoral nail insertion in clinical practice today [7, 13–15]. TAD is the sum of the distances between nail tip and apex point in anteroposte- rior (AP) and lateral views; this risk parameter is also quite convenient, as fuoroscopy efortlessly reveals the two per- taining distances [9]. Consequently, TAD is used widely to assess the safety of the nail inside the femur and to predict whether cut-out is likely to occur [10, 23, 24]. Typically, the insertion of the nail with a TAD below 25 mm is accepted as a stable fxation and low risk of cut-out, albeit challenged to a certain degree by other studies [6, 11, 12]. In some ret- rospective studies, a substantial correlation between TAD and mechanical failure could not be found; factors such as appropriate quadrant insertion and varus position reduction could have bigger infuence in stability [20, 22]. * Ismail Lazoglu ilazoglu@ku.edu.tr 1 Manufacturing and Automation Research Center, Koc University, Istanbul 34450, Turkey 2 Department of Orthopaedics and Traumatology, Koc University Hospital, Istanbul 34365, Turkey