Femoral derotation osteotomy (FDO) is an established surgical procedure to correct rotational bony malalignment of the femur bone and is proven to counteract the typical internal hip rotation gait in patients with cerebral palsy (CP) . The femur is severed by perpendicular osteotomy and both segments are readjusted to enclose a predefined derotation angle on the transversal plane. Fixation is then performed with a plate osteosynthesis. The applied derotation angle is commonly controlled by goniometer and guidance pins (k-wires) which are placed parallel proximal and distal to the planned osteotomy site prior to derotation. However, therapeutic success frequently falls behind expectations: postoperative clinical assessment of passive rotation as well as dynamic assessment by instrumental 3D gait analysis reveal both highly variable over- or undercorrection (deviation to target ≥ ± 10°)  and a systematic loss of correction in a considerable number of patients , .
With today’s availability of sophisticated instrumental gait analysis (achieving measurement repeatability in hip rotation within ±3° standard error ), determination of the proper correction amount has developed to a very high technical standard. Thus, difficulties in maintaining the accuracy in surgical execution have become a growing clinical concern , .
Computer navigated procedures are a frequent approach to improve surgical accuracy, yet none have established in FDO. Many commercially available surgical navigation systems in orthopedics suffer from lack of clinical acceptance due to their large technical footprint , demanding planning requirements, inflexible workflow protocols or custom tool requirements.
In this work, we propose a novel computer assistance approach designed to support FDO with instrumented derotation measurement based on the maturing electromagnetic tracking technology. Our design goal is to provide surgeons with unobtrusive, continuous monitoring of correction parameters, yet without active guidance as in conventional navigation. A prototype system is presented here and used to assess its technical and clinical suitability.
2 Measurement system design
2.1 Parametric model
As distinguished from earlier navigation approaches in this domain , we consider only the local femur anatomy available in the operation site. Derotation is defined as a relative angular change between the proximal and distal femur segment in the transverse plane, perpendicular to the femur axis (see Figure 1). Since rotation about the femur axis in FDO remains largely neutral to the biomechanical axis , neither the location of osteotomy, nor the full bone geometry needs to be defined to achieve proper treatment. While this comes at the cost of uncontrolled degrees of freedom, our model assumptions reflect practical conditions in surgery and help reduce complexity in the assisted procedure. In surgical practice, derotation may deflect the femur axis to a small degree due to unintentional oblique osteotomy. Our preliminary investigations have shown that freehand guidance of the saw between the guide pins is mastered well enough such that the present variability in the cut plane does not significantly impact the biomechanical result .
2.2 System implementation
To implement the measurement system, we chose an electromagnetic tracking (EMT) system (NDI Aurora v.2), which is technically capable of accurate position and orientation measurement in 6 degrees of freedom (6-DOF). Unlike optical tracking, it easily accommodates the tight space constraints in the surgical site and offers a compact installation footprint. For surgical use, sterilizable EMT sensors integrated into screw clamps and a pointer tool for anatomical registration is supplied by fiagon GmbH.
Prior to the OR procedure, the EMT field generator is enclosed in a cushion and placed on the operating table to support the patient’s upper leg (see Figure 2). The system unit and a laptop computer are placed on a trolley nearby.
Emerging from experimental studies , EMT sensors are conveniently attached to the existing k-wires as shown in Figure 3. On the proximal wire, redundant sensors serve as a dynamic reference frame. A sensor on the distal wire keeps track of the bone segment orientation relative to the proximal sensors.
To calculate and monitor the achieved derotation angle, an initial (static) calibration measurement of the assembled sensors and a precise definition of the femur axis are required. Thus, it is essential to first reconstruct the orientation of the local femur axis, as we presented in : We estimate the femur axis from a distribution of 3D points representing the accessible surface of the femur, which is sparsely sampled with the pointer tool prior to osteotomy. Once the bone is osteotomized, the system continuously acquires the orientation transform from the distal sensor relative to the reference base. The derotation angle is then calculated by vector projection onto the transversal plane defined by the femur axis.
3.1 Technical requirements
EMT technology is inherently susceptible to biased measurement induced by ferromagnetic materials in close proximity to the working volume. Recent research shows that technological advancements have increased robustness to a qualifying level for OR applications . Still, precautions need to be taken to ensure reliable operation.
To satisfy a desired derotation measurement accuracy of ±3° (on par with gait analysis), we first assessed the static measurement accuracy in a typical OR setting with a calibration phantom . Orientational error remained within a sub-degree range when the EMT field generator was placed on top of the operating table, remaining in close proximity to the working area.
To detect dynamic measurement error caused by metallic surgical tools, which are inevitably present during the procedure, we first evaluate the hardware-internal NDI error indicator which is carried with every measurement sample from each 6-DoF sensor. Measurement samples with intolerable error values are discarded at the cost of signal availability. Secondly, the redundant reference sensor array delivers an estimate of application accuracy. Since both reference sensors retain a fixed physical geometry during the procedure, any measured disparity in relative orientation compared to an initial calibration indicate presence of dynamic measurement bias and deliver an estimate of the minimum error in derotation measurement.
3.2 Measurement accuracy
To assess the achievable application accuracy of the EMT system in derotation measurement, we carried out FDO on 21 artificial sawbones equipped with EMT sensors on a lab workbench . To address the effects of metallic interference, typical surgical tools (k-wires, Hohmann levers, forceps, fixation plates) were arranged in realistic dimensions of the surgical field. A tripod-mounted camera captured intermediate states in the procedure from a transversal perspective. EMT derotation measurement was finally documented after plate fixation and compared to the change in femur anteversion in baseline and postoperative CT scans, which served as a ground truth.
The average disagreement in the measured derotation angle between our EMT system and CT scans amounted to 0.1° ± 1.6° (mean ± SD) over all sawbones, which certainly falls within the accuracy of CT determination. Measurement distortion during the derotation adjustment was evident to a varying degree in presence of surgical tools. Notably, the steel forceps caused visible disagreement in derotation measurement of EMT and our photographic reference, which was found to be −0.9° ± 2.0° with the forceps present and dropped to −0.3° ± 1.2° without. The created measurement bias, in a typical range of 0.5° up to 3.0°, was also sensed by the hardware-provided error indicator values and the inter-sensor orientation disparity within the reference array.
In contrast, presence of Hohmann levers had no effect on EMT measurements at all. During interventions with saw or drills we observed temporary signal artifacts and limited measurement availability.
3.3 Early clinical experience
Clinical applicability and real-world performance of our EMT prototype is currently assessed in a recently approved pilot trial with CP patients undergoing conventional FDO. Since its reliability in OR conditions is subject of the investigation, EMT derotation measurement is kept hidden from the participating surgeons. The screw clamp sensors were well received among OR staff and do not compromise routine tasks. We recorded an additional time requirement of 5–6 min for the sensor assembly, initial calibration and tactile femur registration.
Maximizing the accuracy of the FDO procedure is of high clinical relevance to improve reproducibility of outcomes, since a significant source of error in the overall therapy process is eliminated. Our prototype measurement system has been designed to meet this particular clinical demand and represents a bottom-up approach to computer assistance in femoral correction surgery, as opposed to planning-driven approaches implementing navigated transfer to the patient. By avoiding additional planning requirements, navigated surgical tools or interventional imaging, it preserves compatibility with the established procedure and habits, offers low effort of use and grants surgeons full autonomy in their actions. The option to enquire the applied derotation angle at any time during the procedure without preparation and time delays is highly requested among orthopaedic surgeons and overcomes an important limitation of the conventional method.
Thanks to the benefits of electromagnetic tracking technology in terms of handling and installation footprint, clinical application is neither invasive nor obstructive to the procedure. Additionally, the system is implemented on readily available tracking hardware and sensor components certified for clinical use. A mentionable limitation in our system setup is the fixed field generator which is opaque to X-rays and has appeared to be inconvenient during occasional control imaging through the operating table. This known drawback can be eliminated by alternative, X-ray compatible field generator designs such as the torus-shaped WindowFG .
Achieving reliable and accurate measurement under OR conditions is crucial for real-world applicability and poses a challenge to EMT technology. Application accuracy in our phantom study was promising as measurement errors did not exceed clinically relevant dimensions even in the presence of standard surgical tools . Nevertheless, our lab setup cannot fully simulate OR conditions and limits transferability of result. An ongoing clinical trial will reveal if reliable measurement operation can be upheld under clinical conditions. Measurement error detected by the system appears to be comparable to the observations in our lab study, but has yet to be evaluated in follow-up CT control. Furthermore, surgical tools made from specific steel alloys compatible to EM tracking have shown to substantially reduce field distortion  and could be considered for future application.
Taking a broader view, femoral correction procedures are not limited to rotation adjustment. Complex multiplanar adjustments with double osteotomies create many degrees of freedom and are even more difficult to control in the conventional technique. To exploit the full potential of computer assistance in such scenarios, both orientational and positional adjustments must be accurately defined and transferred to the patient, which typically leads to planning overhead and requires multi-step registration routines as well as active tool guidance. Belei et al.  developed an interventional planning, biomechanical optimization and navigation method for multiple femoral correction procedures controlling every possible degree of freedom. Planning transfer was accomplished by anatomical registration in calibrated X-ray imaging, navigated tools and mechanical alignment templates. Despite the accurate performance demonstrated in lab studies, invasive fixation of optical trackers remained an unsolved problem.
To establish a wider scope of application without burdening surgeons with unacceptable complexity, we consider future development of customized sensor designs and anatomical sampling procedures for each individual type of correction as well as integration of available biomechanical model data from 3D gait analysis.
Research funding: The author state no funding involved. Conflict of interest: Authors state no conflict of interest. Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance with the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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