- Research article
- Open Access
- Open Peer Review
Improved lag screw positioning in the treatment of proximal femur fractures using a novel computer assisted surgery method: a cadaveric study
© Regling et al.; licensee BioMed Central Ltd. 2014
- Received: 1 November 2013
- Accepted: 14 May 2014
- Published: 30 May 2014
The importance of the tip-apex distance (TAD) to predict the cut-out risk of fixed angle hip implants has been widely discussed in the scientific literature. Intra-operative determination of TAD is difficult and can be hampered by image quality, body habitus, and image projection. The purpose of this paper is to evaluate, through a cadaveric study, a novel computer assisted surgery system (ADAPT), which is intended for intraoperative optimisation of lag screw positioning during antegrade femoral nailing. A 3D measure for optimal lag screw position, the tip-to-head-surface distance (TSD), is introduced.
45 intra-medullary hip screw procedures were performed by experienced and less experienced surgeons in a cadaveric test series: in 23 surgeries the ADAPT system was used, and in 22 it was not used. The position of the lag screw within the femoral head and neck was evaluated using post-operative CT scans. TAD, TSD, fluoroscopy as well as procedure time and variability were assessed.
The use of the ADAPT system increased accuracy in TSD values (i.e. smaller variability around the target value) for both groups of surgeons (interquartile range (IQR) of experienced surgeons: 4.10 mm (Conventional) vs. 1.35 mm (ADAPT) (p = 0.004)/IQR of less experienced surgeons: 3.60 mm (Conventional) vs. 0.85 mm (ADAPT) (p = 0.002)). The accuracy gain in TAD values did not prove to be significant in the grouped analysis (p = 0.269 for experienced surgeons; p = 0.066 for less experienced surgeons); however, the overall analysis showed a significant increase in accuracy (IQR: 4.50 mm (Conventional) vs. 2.00 mm (ADAPT) (p = 0.042)). The fluoroscopy time was significantly decreased by the use of the ADAPT system with a median value of 29.00 seconds (Conventional) vs. 17.00 seconds (ADAPT) for the less experienced surgeons (p = 0.046). There was no statistically significant impact on the procedure time (p = 0.739).
The ADAPT system improved the position of the lag screw within the femoral head, regardless of the surgeon’s level of clinical experience, and at the same time decreased overall fluoroscopy usage. These positive effects are achieved without increasing procedure time.
- Computer assisted surgery
- Lag screw placement
- Proximal femur fractures
- Hip fractures
- Surgical technique
Hip screw cut-out with penetration into the hip joint has been reported to be one of the major complications in the treatment of per-trochanteric hip fractures with fixed angle devices. The occurrence of this complication still ranges from 1.2-8.5% with sliding hip screws and intramedullary nails in recent studies [1–17], although significant improvements in the surgical technique have already led to a decrease in cut-out rates . In earlier studies, its occurrence has been reported to be as high as 12.6-16% [18, 19]. Once cut-out has occurred, the patient typically faces difficult reconstructive options, often leaving no other recourse than conversion to total hip replacement .
In 1995, Baumgaertner et al. introduced the concept of the tip-apex distance for predicting the risk of failure of fixation by lag screw cut-out . They demonstrated that increasing TAD above 25 mm was strongly correlated with an increased risk of lag screw cut-out through the femoral head. Several other studies supported this conclusion, showing that the TAD is a highly significant predictor of mechanical failure due to cut-out [10, 12, 14, 15, 21, 22]. In a later study, Pervez et al. recommended a TAD of less than 20 mm . Besides the TAD, the position of the lag screw within the femoral head as described by Parker in 1992 has been identified to influence cut-out , with the optimal positioning of the screw remaining controversial. While numerous studies found the centre-centre position in the AP and lateral planes to be most advantageous [18, 20, 24–26], many authors of both biomechanical as well as clinical studies recommend placing the lag screw in the inferior half of the femoral head in the antero-posterior (AP) view and in the centre of the femoral head in the lateral view [14, 17, 27–33].
The purpose of the present paper is to introduce and evaluate a computer-assisted surgery (CAS) method that assists the surgeon in accurately positioning the tip of the screw intra-operatively in real time, independent of the position of the lag screw relative to the centre-centre axis of the femoral head. The technique and results of a cadaveric series are presented.
The ADAPT system
Excursion: the difference between the TAD (Tip-Apex Distance) and the TSD (Tip-to-head-Surface Distance)
The tip-to-head-surface distance (TSD) is a concept for a 3D measurement of the 3D distance of the tip of the lag screw to the surface of the femoral head in direction of the lag screw axis.
Cadaveric test series using the ADAPT system
Two cadaveric tests were conducted to assess the impact of the ADAPT system on the lag screw placement during a Gamma3 surgery. The cadaveric tests were performed at the Texas Health Research & Education Institute in Dallas (TX), USA on September 9th - 10th, 2010, and on April 4th - 5th, 2011 at the Academy for Medical Training and Simulation in Lucerne, Switzerland. Ethics approval was not required in the USA and Switzerland for cadaveric studies as per federal laws. However, the cadaveric test performed in Dallas was approved by the Anatomical Board of the State of Texas and the institute in Lucerne is in full compliance with medical-ethical guidelines and recommendations of the Swiss Academy of Medical Sciences. Consent for the storage and use of the bodies for research purposes was given by all body donors prior to death or by their next of kin.
Experienced surgeons, who perform more than 50 Gamma3 surgeries per year, as well as less experienced surgeons, who perform less than 15 Gamma3 surgeries per year, participated in the cadaveric test series. All surgeons received product training of the ADAPT system, and less experienced surgeons attended a Gamma3 workshop on sawbones.
Case processing summary
Following this study setup, the participating surgeons were split into four different groups: (1) experienced surgeons using ADAPT, (2) experienced surgeons using conventional fluoroscopy, (3) less experienced surgeons using ADAPT and (4) less experienced surgeons using conventional fluoroscopy.
The surgeons were asked to attempt to position the lag screw in their best estimate of centre-centre position with a distance of 5 mm from the surface of the femoral head (TSD).
The real 3D position of the lag screw within the femoral head was assessed using post-operative CT-scans; no problems with artefacts were encountered during the assessment. The evaluation of the CT-scans was performed with OrthoMap 2.0-19; the reviewer was blinded to sample and method of placement. C-arm images were used to post-operatively measure the TAD. Only the procedural steps that are supported by the ADAPT system were timed (e.g. excluding time needed for patient positioning, incision) and compared to avoid biasing factors. Fluoroscopy times were measured by the C-arm. During analysis and group comparison the statistician was blinded with regard to the identity of the group.
The parameters of interest for the statistical analysis were the accuracy of the lag screw placement as measured with TSD and TAD, the procedure time and the fluoroscopy time. The results were analysed for descriptive statistics with focus on the central position and variation measures. Quantitative data was assessed for normality by using the Shapiro-Wilk test. Due to small subgroup sample sizes and the absence of normality, non-parametric tests were applied for the inferential statistics. In order to assess the differences in accuracy between the groups, the variability of values was analysed by means of the Moses test. Procedure time and fluoroscopy time were examined with the Mann–Whitney test. The robustness of both tests was increased by the application of a Monte Carlo Simulation with 10.000 runs. The significance level for all tests was set at 95% (alpha = 0.05). All statistical analyses were performed using SPSS/PASV V.17.
Accuracy - ADAPT vs. conventional technique
Tip-to-head-Surface Distance (TSD)
Accuracy TSD - ADAPT vs. conventional technique - descriptive statistics
Interquartile range [mm]
Interquartile range [mm]
Tip-apex distance (TAD)
Accuracy TAD - ADAPT vs. conventional technique - descriptive statistics
Interquartile range [mm]
Interquartile range [mm]
Procedure time - ADAPT vs. conventional technique
Procedure time - ADAPT vs. conventional technique - descriptive statistics (only less experienced surgeons)
Interquartile range (hh:mm:ss)
Fluoroscopy time - ADAPT vs. conventional technique
Fluoroscopy time - ADAPT vs. conventional technique - descriptive statistics (only less experienced surgeons)
Interquartile range (seconds)
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.
The first experiences with the ADAPT system gained through a cadaveric test series show that both experienced as well as less experienced surgeons can benefit from the ADAPT system through more accurate lag screw placement. The system uses existing equipment and smoothly integrates into the surgical workflow, does not increase procedure time, but statistically decreases the fluoroscopy time. Especially less experienced surgeons can benefit from the system and it can be a useful training tool. However, first experiences are limited to cadaveric tests thus far. Further experiences should be made in clinical settings. Still, the first results seem to be very promising.
The authors would like to thank Claudia Beimel for performing the statistical analysis.
We thank the following surgeons for participating in the cadaveric test series (in alphabetical order):
Dallas, TX, USA: James W. Maxey, MD; Robert A. Probe, MD; Paul D. Saadi, MD; Brian D. Solberg, MD.
Lucerne, Switzerland: Dr. Mai Lan Dao Trong; Dr. Arthur Grzesiak; Dr. Stefanie-Peggy Kuehnel; Dr. Kerstin Schneider.
Moreover, we thank the personnel at the Texas Health Research & Education Institute in Dallas (TX), USA as well as at the Academy for Medical Training and Simulation in Lucerne, Switzerland.
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