Abstract
Background: Despite considerable recent interest in computer navigation for orthopaedic surgery, few investigations of computer-assisted surgery for foot and ankle operations have been reported. The purpose of the present study was to compare subtalar arthrodesis with and without computer navigation in a cadaver model.
Methods: Subtalar arthrodesis was performed on thirty-six matched-pair cadaver lower extremities with intact soft tissues, with an attempt being made to orient two screws in the optimal configuration based on unpublished data from a preceding biomechanical study. Each matched pair was randomly assigned either to a group of surgeons who were experienced in subtalar arthrodesis or to a group of inexperienced operators. Neither surgical group was experienced in computer-assisted surgery. We compared optimal first-pass guidewire placement, fluoroscopic time, total operative time, screw placement accuracy, and adverse screw placement events between conventional (fluoroscopically guided) and computer-assisted subtalar arthrodesis.
Results: The number of passes needed to achieve optimal guidewire placement decreased with the use of computer assistance for both experienced surgeons and inexperienced operators (p < 0.001), with ideal placement occurring on the first attempt in 95% of the procedures performed with use of computer assistance. While the experienced surgeons required less time and fewer guidewire passes during conventional subtalar arthrodesis than the inexperienced operators did (p < 0.001), both groups used less fluoroscopy with computer assistance (p < 0.001). There was no significant difference in operative time between the two techniques when performed by the inexperienced operators, yet the total procedure time doubled for the experienced surgeons when the procedure was performed with use of computer assistance (p < 0.001). There was no significant difference between experienced surgeons and inexperienced operators or between conventional and computer-assisted subtalar arthrodesis with respect to adverse screw placement events or the ability to accurately place both screws.
Conclusions: Computer-assisted subtalar arthrodesis resulted in screw placement accuracy that was equivalent to that of conventional (fluoroscopically guided) subtalar arthrodesis while decreasing the number of suboptimal guidewire passes and fluoroscopic time. The computer-assisted surgery technique increased the operative time for surgeons who were more experienced in conventional subtalar arthrodesis, but there was no difference in operative time for the group of operators who were inexperienced in subtalar arthrodesis.
Clinical Relevance: The present study supports the use of computer assistance for subtalar arthrodesis, particularly for surgeons who are less experienced in the procedure.
Computer-assisted surgery has been used for various orthopaedic procedures since the early 1990s1-3. This technology is being employed with greater frequency because of improved visualization, greater accuracy of implant placement, and reduced radiation exposure as compared with conventional orthopaedic surgery. While computer-assisted surgery techniques have been used for hip and knee replacement1,4-12, sports medicine procedures13-15, spine surgery16, and orthopaedic trauma surgery17-21, we are aware of few reports that have described the use of these techniques for the treatment of foot and ankle disorders22-25.
Subtalar joint arthrodesis is a relatively common foot and ankle procedure that is performed for the treatment of primary and secondary subtalar arthritis, late-stage posterior tibial tendon insufficiency, and select cases of tarsal coalition26-30. Typically, calcaneus-to-talus screw fixation is recommended27-33. Optimal screw placement for subtalar arthrodesis is important in order to minimize intertarsal motion and to potentially increase the likelihood of fusion. On the basis of unpublished data from a previous biomechanical investigation, we believe that a double diverging screw configuration affords superior stability and compression for subtalar arthrodesis when compared with two parallel screws or several single screw configurations. Specifically, for the present investigation, optimal screw placement was defined as one screw beginning in the posterolateral calcaneal tuberosity and ending in the centrolateral talar body and a second screw starting in the posterocentral calcaneal tuberosity and terminating in the head-neck junction of the talus. Placing two arthrodesis screws in such a configuration, while simultaneously controlling heel position, may prove to be difficult with use of intraoperative fluoroscopy alone. The fluoroscope must be manipulated in at least two planes as the surgeon orients and stabilizes the hindfoot to pass screws (or guidewires for cannulated systems) across the subtalar joint. Although an experienced surgeon occasionally may need only a single pass of a drill-bit or guidewire, we have noted that a less experienced surgeon typically requires more than one pass to achieve the desired implant position. Multiple attempts to reposition the screw or guidewire may lead to weakening of the osseous structure, thereby diminishing screw purchase and compression and potentially increasing the risk of nonunion. In addition, creating numerous channels in the bone with each pass attempt increases the difficulty of establishing a new, more accurate path for the guidewire. Finally, the required multiple-shot or real-time fluoroscopy exposes the patient, surgeon, and hospital staff to more radiation.
Computer-assisted surgery has been shown to improve the accuracy of implant positioning in other orthopaedic procedures while maximizing operative efficiency and minimizing bone destruction and radiation exposure12,13,18,21,34,35. We hypothesized that computer-assisted surgery could enhance the first-pass accuracy of guidewire and screw placement for subtalar arthrodesis. We designed the present investigation to compare screw placement accuracy, the number of attempts to achieve the desired screw position, and the surgical and fluoroscopic times in computer-assisted and conventional (fluoroscopically guided) subtalar arthrodesis with use of a cadaver model.
Technique
Thirty-six matched pairs of fresh-frozen human cadaver lower extremities were used in the present study. Calcaneal bone mineral density was measured with dual x-ray absorptiometry with use of the Lunar PIXImus Bone Densitometer (version 1.44; Lunar PIXImus, Fitchburg, Wisconsin) to quantify bone quality36. Matched-pair specimens were randomized to either an experienced surgeon group or an inexperienced operator group and then were assigned to a particular surgeon or operator within each group. Unpaired limbs were randomly selected for a technique, with the only consideration being given to laterality so that an equal number of right and left feet were used for each technique per surgeon or operator. A total of six individuals (M.E., B.C., J.R., N.C., R.S., and I.L.D.L.), including three experienced surgeons and three inexperienced operators, operated on no fewer than four feet per technique; in the initial part of this investigation, one experienced surgeon and one inexperienced operator (B.C. and I.L.D.L.) performed each technique on ten specimens. The limb was positioned as it would be during a procedure on a live patient, and a standard approach to the subtalar joint was performed on each extremity without removal of articular cartilage or preparation of subchondral bone. The subtalar arthrodesis was carried out with use of two diverging 7.3-mm cannulated lag screws (Synthes, West Chester, Pennsylvania) in the double-screw configuration described above. The first screw was inserted in the posterolateral calcaneal tuberosity and ended in the centrolateral talar body; the second screw was started in the posterocentral calcaneal tuberosity and terminated in the head-neck junction of the talus. Joint preparation, although a clinically necessary step, was intentionally omitted from our investigation in order to isolate a comparison of the influence of the technology on screw placement and to eliminate anticipated discrepancies between the experienced surgeon and the inexperienced operator.
In Group 1 (Conventional—Inexperienced), three inexperienced operators performed a conventional subtalar arthrodesis (with the exception of joint decortication) through a lateral approach with fluoroscopically guided screw placement. The three operators included one orthopaedic surgeon in training without previous experience in subtalar arthrodesis and two medical students with limited or no previous surgical experience. In Group 2 (Conventional—Experienced), three experienced fellowship-trained foot and ankle orthopaedic surgeons performed the same conventional surgery as described for Group 1.
In Group 3 (Computer-Assisted Surgery—Inexperienced), which involved the contralateral limbs matched to those used in Group 1, a subtalar arthrodesis was performed (again with the exception of joint decortication) by an operator who was inexperienced in the procedure with use of the BrainLAB VectorVision computer-assisted surgery system (BrainLAB, Feldkirchen, Germany). The navigation system comprises a camera unit, standard orthopaedic instruments with reflective reference arrays, and a computer system running Trauma Navigation Software 2.6 (BrainLAB). Through a standard lateral approach to the subtalar joint, the talar tracking array anchor (a single specialized 3.0-mm threaded pin) was secured to a nonarticular portion of the talar head-neck junction by means of bicortical fixation. The same array is routinely used for computer-assisted total knee arthroplasty. One anteroposterior and one lateral fluoroscopic image were made to confirm satisfactory placement of the tracking array anchor (Fig. 1-A). The talar array was then secured with a hexagonal locking nut to the specialized pin. As the locking nut was tightened down, the four low-profile teeth at the base of the array anchoring system pierced the cortex of the lateral aspect of the talar neck, becoming fixed to the talus. Before the procedure was continued, satisfactory stability of the array apparatus on the talus was confirmed manually, as would be done for an array assembly in the tibia or femur for total knee arthroplasty, to ensure that the array and the talus moved in unison (Fig. 1-B). The stability of the array on the talus was again confirmed at the conclusion of each computer-assisted surgery procedure. Care was taken to stabilize the subtalar joint in the optimal, recommended position with the ankle in neutral dorsiflexion-plantar flexion and neutral or slight valgus and by placing the calcaneus in 5° of valgus. The subtalar joint was then provisionally fixed with a 2.0-mm Kirschner wire that was passed from the tuberosity of the calcaneus into the talar body. Four fluoroscopic views of the foot were made and co-registered to the VectorVision guidance computer. While the BrainLAB guidance software provides certain checks and filters for the quality of the transferred fluoroscopic image, it is entirely incumbent on the operator to obtain the necessary, accurate, and properly oriented views for guidance registration. Prior to the commencement of all procedures, the senior experienced surgeon (M.E.) provided basic radiographic instruction to the inexperienced group and demonstrated the acquisition and registration process for the lateral foot, anteroposterior foot, ankle mortise, and Harris heel views that were required for both the conventional and computer-assisted surgery techniques. During the procedures, none of the inexperienced operators received further instruction. The infrared camera tracked the talar array along with reference arrays on the fluoroscope's C-arm and a specialized drill-guide, transmitting orientation and positional data for the foot and guidewire path to the VectorVision planning screen, and superimposing this information onto the four aforementioned fluoroscopic views in real time (Fig. 2). The screw paths were planned on the touch-screen display, and intended trajectories were determined with real-time simultaneous targeting of the guidewire in all four planes and were displayed on the VectorVision monitor. In Group 4 (Computer-Assisted Surgery—Experienced), computer-assisted subtalar arthrodesis was performed, in exactly the same manner as described earlier for Group 3, on the contralateral limbs matched to those used in Group 2, by the same experienced foot and ankle specialists who performed the procedures in Group 2. None of the investigators had previous experience with computer-assisted surgery for orthopaedic procedures.
Intraoperative Data Collection
During each procedure, five measurements were recorded: (1) the elapsed time from incision until completion of the arthrodesis (total procedure time), (2) the additional time required for the computer-assisted-surgery-specific steps of image registration, array placement, and provisional fixation of the subtalar joint (time for computer-assisted-surgery-specific steps), (3) the time required for guidewire insertion and screw placement (screw placement time), (4) the duration of radiation exposure (fluoroscopic time), and (5) the number of attempts required to insert the guidewires in the desired orientation. A standard approach to the subtalar joint was made in each specimen; the approach time to the subtalar joint was included in the calculation of the total procedure time.
Postoperative Radiographic Assessment
Anteroposterior and lateral foot, calcaneal axial (Harris heel), and ankle mortise fluoroscopic images were made on completion of the procedure. The initial twenty matched-pair specimens were disarticulated at the ankle, and the superior talar surface was first carefully inspected for adverse penetration by the implants. Subsequent to this inspection, screws were intentionally advanced through the superior talar surface to act as a reference for making anatomical measurements of final screw tip positions in the sagittal, coronal, and axial planes with use of a caliper. In order to preserve cadaver tissue for future unrelated study and to exercise the judicious use of these specimens, computed tomography scans with a 0.625-mm thickness with three-dimensional reformatting were made for the remaining sixteen matched-pair specimens. For completeness and consistency, disarticulation was also performed on three matched pairs that underwent computed tomography imaging, and computed tomography scans were made for three matched pairs that were previously dissected.
To quantify screw placement accuracy, the final fluoroscopic images were compared with either the computed tomography scans or the measurements that were obtained through dissection by determining the relative position of the screw tip to osseous talar landmarks (Fig. 3). A similar method was employed in a previously published navigation study of femoral neck fractures21. First, we calculated the percentage difference between the intended screw tip position (as determined fluoroscopically) and the final screw tip position (as measured on the specimens or with computed tomography). Second, we calculated average talar axes across all specimens in the sagittal, coronal, and axial planes and compared these values with the value of one screw diameter (7.3 mm) to determine the percent fraction of one screw width in each of the three talar dimensions. By comparing the first and second values, we derived the screw accuracy percentage offset to assess screw placement accuracy. A screw was deemed to be accurately placed when the final screw tip position fell within one screw diameter from where it was intended. From the six matched-pair specimens that underwent both anatomic dissection and computed tomography scanning, the calculation of screw accuracy percentage offset was equivalent when based on either the anatomic caliper measurements or computed tomography imaging. Adverse screw placement was defined as penetration of the calcaneal or talar cortices, penetration of the ankle joint by the screws, or clinically suboptimal screw positioning.
Postoperative evaluation of the computer-assisted surgery specimens was made in exactly the same manner as described above, with the only exception being that, for the radiographic analysis, BrainLAB navigation planning screenshots that had been obtained during the procedure (Fig. 2) were compared with the anatomic caliper or computed tomography measurements. Both the radiographic findings and the surgical specimen examinations were confirmed by another investigator (T.O.) who was not involved in performing the procedures.
Statistical Methods
Comparisons among the four experimental groups were performed with use of two-by-two repeated mixed-measures analysis of variance and Tukey post hoc analysis with an alpha level of 0.05.
A comparison of surgical technique measurements is presented in Table I. The mean number of passes required to achieve optimal guidewire placement decreased with computer-assisted surgery for both groups (p < 0.001), with desired placement occurring on the first insertion in 95% of the computer-assisted procedures. The mean fluoroscopic time for all surgeons and operators was reduced by 87% with the use of computer-assisted surgery (p < 0.001).
During conventional subtalar arthrodesis, the experienced surgeons required less time for completion of screw placement (p < 0.001), less time for completion of the procedure (p < 0.001), and fewer guidewire placement attempts (p < 0.001) than the inexperienced operators did. For the inexperienced operators, computer assistance facilitated a 42% (10.4-minute) reduction in mean screw placement time (p < 0.001), with no significant change in mean total operative time as compared with conventional subtalar arthrodesis. For the experienced surgeons, computer assistance increased the mean screw placement time by 42% (5.7 minutes) (p < 0.01) and increased the mean total procedure time by 118% (17.2 minutes) (p < 0.001). The inexperienced operators required less time for screw placement (p < 0.01) and procedure completion (p < 0.05) with computer-assisted surgery as compared with the experienced surgeons. Both groups completed computer-assisted-surgery-specific steps, array placement, image acquisition, and provisional fixation of the subtalar joint in the same amount of time.
A screw placement accuracy rate of 99.3% was achieved for all procedures, irrespective of technique or experience level (Table II). There was no significant difference between conventional and computer-assisted surgery techniques with respect to either the experienced surgeons' or the inexperienced operators' ability to accurately place both screws or their respective adverse placement rates (Tables I and II). Overall, the adverse screw placement rate was 5.6% for the conventional technique and 6.9% for the computer-assisted surgery technique (Table II). Although the difference was not significant, there was an 8.3% increase in the adverse placement rate for the experienced surgeons in association with the use of the computer-assisted surgery technique.
In the computer-assisted surgery group, one screw terminated immediately lateral to the talar neck and was detected on computed tomography and dissection of the specimen. A detailed review of this case revealed that the intraoperative anteroposterior fluoroscopic image of the foot that had been made and registered to the navigation computer was suboptimal. Although the screw navigation had been performed properly, it had been done with reference to this poorly acquired image. Three (8%) of the thirty-six medial screws that were inserted during the computer-assisted surgery procedures and four (11%) of the thirty-six medial screws that were inserted during the conventional procedures penetrated the medial calcaneal cortex. One (2.8%) of the thirty-six lateral (talar dome) screws that were inserted during the computer-assisted surgery procedures penetrated the superior talar dome. A review of the corresponding registered images revealed that the screw length had been inappropriately determined on the basis of a rotated lateral view; incidentally, this was also the single case in which the array anchor inadvertently contacted the C-arm during image acquisition.
Bone mineral density measurements showed a statistical consistency of bone quality across all specimens, with an average value (and standard deviation) of 0.5517 ± 0.1353 g/cm2.
To our knowledge, the present investigation is the first to objectively compare conventional and computer-assisted techniques for orthopaedic foot and ankle surgery. We employed a systematic method to compare intraoperative measures for subtalar arthrodesis, a relatively common foot and ankle procedure. In conventional subtalar arthrodesis, intermittent fluoroscopic images are made to confirm proper guidewire, drill-bit, and screw trajectory and position throughout the procedure. In contrast, the computer-assisted surgery technology that was evaluated in the present investigation relies exclusively on four fluoroscopic images that are made prior to guidewire or screw insertion. During guidewire, drill-bit, and screw placement, no confirmatory fluoroscopic images are made until the final screw position has been achieved. We had no previous experience with computer navigation for orthopaedic surgery.
Our investigation confirms high screw placement accuracy in association with the use of conventional fluoroscopic techniques for subtalar arthrodesis. We observed similar screw placement accuracy in association with computer navigation. Our results indicate that the computer-assisted technique for subtalar arthrodesis may offer advantages over conventional subtalar arthrodesis, especially for a surgeon who has limited experience with the procedure. Specifically, computer-assisted surgery technology increased first-pass guidewire placement accuracy, decreased the time needed to place guidewires or screws in the desired orientation, and diminished radiation exposure in comparison with the conventional technique. Reduced integrity of thread purchase with final screw placement, sometimes associated with multiple guidewire or drill passes, can be an issue, particularly in osteoporotic patients37 and those undergoing revision surgery. Improving first-pass accuracy for guidewire or screw placement can minimize this risk. Moreover, the surgeon's ability to place the guidewire, drill-bit, or screw in the desired position may be compromised by the creation of multiple channels that are difficult to avoid in subsequent passes.
The quality of the result, namely, the ability to accurately place both screws, was equivalent for the two experience levels, regardless of technique, and the experienced surgeons benefited equally from the reduced number of guidewire placement attempts and from the reduced fluoroscopy time. Our findings demonstrate that computer-assisted surgery can assist an inexperienced surgeon, even one with limited or no previous surgical experience, to place screws in roughly half the time needed with use of the conventional method. Furthermore, we demonstrated that an experienced surgeon who has the ability to optimally place guidewires conventionally can do so with fewer passes with use of computer-assisted surgery.
Screw placement time and overall computer-assisted surgery time for the experienced surgeons exceeded those required for conventional surgery and, unexpectedly, also exceeded the time required for computer-assisted arthrodesis by the inexperienced operators. We attribute the additional time required by experienced surgeons for the performance of computer-assisted subtalar arthrodesis to a unique learning curve, one in which the experienced surgeon transitions from learned methods of screw placement to new ones that demand not only trusting the computer guidance but also learning a new sequence of steps to perform the arthrodesis. In contrast, an operator who is unfamiliar both with conventional methods and with navigation procedures for screw placement in subtalar arthrodesis faces no such conflict. Both techniques are equally novel to the inexperienced operator, as reflected by the nearly equal completion times with use of either method. We recognize that if this study were to be repeated in the clinical setting for subtalar arthrodesis, with the inclusion of comprehensive joint preparation, the total procedure times would likely be longer, particularly for the inexperienced surgeon. This clinically necessary step was omitted from our cadaver investigation in order to eliminate the variable of joint preparation and to create an equal footing on which to compare the technology's influence on screw placement between the two experience groups.
The accuracy of final screw tip positioning in the computer-assisted surgery group (98.6%; seventy-one of seventy-two navigated screws) was comparable with the rate achieved in the conventional group (Table II). We surmise that the risk of an undesirable final screw position is increased if the foot is not optimally aligned in the four fluoroscopic views that become the acquired static images used by the surgeon in planning screw trajectories. Penetration of the lateral talar neck cortex occurred in association with one (1.4%) of seventy-two navigated screws; we attributed the inaccurate screw trajectory to poor image acquisition. Penetration of the superior talar dome occurred in association with one (2.8%) of thirty-six lateral (talar dome) navigated screws. In a careful review of these two cases, we identified two potential technical errors: (1) inadvertent compromise of the array's anchoring system to the talus and (2) a suboptimal fluoroscopic image used in planning.
Despite the achievement of accurate final screw tip positions, we observed the following adverse events. Three of the thirty-six computer-navigated medial screws and four of the thirty-six conventionally positioned medial screws penetrated the medial calcaneal cortex, with all seven of these screws exhibiting accurate final screw tip position in the talar head-neck junction. Medial calcaneal cortical screw penetration during subtalar arthrodesis is not attributable to limitations of the computer-assisted surgery technology. While starting the medial screw lateral to the midline of the calcaneal tuberosity typically avoids medial cortical penetration27,28,32,38, accurate final screw tip position does not eliminate the risk of screw penetration of the medial calcaneal cortex associated with either the conventional or the computer-assisted surgery technique. Unfortunately, seemingly accurate Harris heel fluoroscopic images with a well-positioned guide pin (conventional technique) or a well-planned screw trajectory (computer-assisted surgery technique) do not ensure screw placement fully contained within the calcaneus.
While reference array instability may cause inaccuracies with the navigation system and consequently a poor screw placement result, we experienced only one such instance in association with the use of a standard long bone array that was secured to the talus. We measured bone mineral density and found that there was no significant difference among the specimens. However, we consider the long bone anchor to be rather cumbersome for a foot and ankle procedure and believe that it is at risk for being dislodged by inadvertent contact, as occurred in one case in the present study. We also recognize that the stress riser created in the talar neck by this anchoring system could pose a potential complication. However, given that a patient typically maintains a non-weight-bearing status for six to eight weeks following subtalar arthrodesis, there is, in our opinion, little concern for the potential formation of a talar neck stress fracture resulting from the 3.0-mm pin utilized in this investigation. A similar cortical defect, or one of even greater size, is commonly created in a similar location in association with the use of external fixators that are employed for other orthopaedic foot and ankle procedures. To our knowledge, there is no reported contraindication to the use of external fixation pins in the talar neck because of a potential resultant stress fracture. Moreover, we expect that as navigation technology evolves for foot and ankle surgery, lower-profile arrays with smaller pin diameters dedicated to small bone and joint navigation will be introduced to further mitigate these concerns.
The present investigation confirms that computer navigation can accurately determine ideal screw position and length in the majority of cases. First-pass accuracy for proper screw trajectory represents the greatest advantage of computer-assisted surgery over the conventional technique. In our opinion, minimal additional fluoroscopic exposure combined with depth gauge assessment could confirm the navigated screw length prior to screw insertion, still resulting in less total radiation than is the case with the conventional technique.
The high cost of the computer-assisted surgery systems may delay the widespread implementation of this technology for subtalar arthrodesis despite the demonstrated potential advantages. The investment cost can, however, be distributed across surgical applications for larger departments and over time may prove to be cost-effective if navigation is shown to improve clinical outcomes. We anticipate that the present investigation will prompt further analysis of computer navigation for orthopaedic foot and ankle procedures, particularly a prospective, randomized clinical trial comparing surgical outcomes between conventional and computer-assisted techniques.
In conclusion, the use of computer-assisted surgery for subtalar arthrodesis decreases the number of suboptimal guidewire passes and fluoroscopic radiation exposure while maintaining a screw placement accuracy rate that is equivalent to the rate associated with conventional subtalar arthrodesis. Computer-assisted surgery may increase the operative time for surgeons who are more experienced in conventional subtalar arthrodesis until they become competent with computer-assisted surgery technology. Care needs to be taken to ensure the quality of fluoroscopic images obtained and registered to the navigation system, as these images constitute the underlying point of reference for accurate navigation. This technology is promising for use in subtalar arthrodesis but will require further refinement before it is ready for clinical use. 
Note: The authors thank Robin M. Queen, PhD, for her help with statistical analysis, and Richard Glisson, BS, for his technical assistance with this manuscript.
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