Radiation- and reference base-free navigation procedure for placement of instruments and implants: Application to retrograde drilling of osteochondral lesions of the knee joint

Abstract

Objective: A novel, radiation- and reference base-free procedure for placement of navigated instruments and implants was developed and its practicability and precision in retrograde drillings evaluated in an experimental setting.

Materials and Methods: Two different guidance techniques were used: One experimental group was operated on using the radiation- and reference base-free navigation technique (Fluoro Free), and the control group was operated on using standard fluoroscopy for guidance. For each group, 12 core decompressions were simulated by retrograde drillings in different artificial femurs following arthroscopic determination of the osteochondral lesions. The final guide-wire position was evaluated by postoperative CT analysis using vector calculation.

Results: High precision was achieved in both groups, but operating time was significantly reduced in the navigated group as compared to the control group. This was due to a 100% first-pass accuracy of drilling in the navigated group; in the control group a mean of 2.5 correction maneuvers per drilling were necessary. Additionally, the procedure was free of radiation in the navigated group, whereas 17.2 seconds of radiation exposure time were measured in the fluoroscopy-guided group.

Conclusion: The developed Fluoro Free procedure is a promising and simplified approach to navigating different instruments as well as implants in relation to visually or tactilely placed pointers or objects without the need for radiation exposure or invasive fixation of a dynamic reference base in the bone.

• Computer assisted surgery
• Fluoro Free navigation
• osteochondral lesion
• retrograde drilling
• radiation free
• pinless
• arthroscopy

Introduction

Technical background: navigation

Positioning of surgical instruments without direct visual control usually requires the use of imaging data, which is mostly obtained by fluoroscopy. With conventional fluoroscopic imaging in only one plane at a time, three-dimensional (3D) imaginative ability is mandatory for the surgeon if exact positioning is to be achieved.

Correction of instrument movements based on a two-dimensional (2D) image can lead to deviation in the third plane which is not visible. Therefore, repeated changes in the fluoroscope position are necessary to control the 3D orientation of instruments. This procedure was described by Kahler et al. as “trial and error” [1], and can lead to high radiation exposure for both the patient and the operating team, especially when working in regions whose complex anatomy makes secure positioning difficult [2–4], or when small targets like osteochondral lesions (OCLs) have to be reached by various instruments [5],[6]. Other complications, such as intra-articular penetrations and iatrogenic fractures, have been reported, especially after multiple changes of instrument position [7–9].

As a result of these known shortcomings of fluoroscopy, especially in anatomically complex osseous regions [2],[3], CT-based computer assisted procedures were introduced. With these approaches, improved precision could be demonstrated for selected procedures [10]. However, due to the unfamiliar techniques, high costs and demanding logistics associated with CT-based navigation, it was demanded that developmental priority be given to fluoroscopy-based 2D and 3D navigation [11],[12]. All of these navigated techniques require pre- or intraoperative imaging and fixation of an invasive reference base in the bone for localization of instruments relative to the patient anatomy. The fixation of such reference bases results in additional violation of the soft tissue and bone, which may induce iatrogenic fractures [13].

To overcome these drawbacks, we developed a radiation-free navigation procedure that is technically based on established 2D fluoroscopic navigation procedures [1],[4],[14]. For this procedure, neither image acquisition nor additional invasive access to the bone for fixation of reference bases is required. The purpose of this study was to evaluate the feasibility and precision of this navigation procedure, as exemplarily applied to retrograde drilling of osteochondral lesions localized by arthroscopy.

Clinical background

Osteochondritis dissecans is a lesion of subchondral bone which can result in sequestration of osteochondral fragments, a known risk factor for secondary arthritis. Although there is no standardized therapeutic concept, it is agreed that therapy should be based on the stage of the lesion. In stage 2 lesions [15],[16] with subchondral sclerosis, conservative therapy is initially recommended, but with a limited duration in adults of six weeks without weight bearing. If there is no benefit from this treatment approach, operative therapy is recommended [17]. Although both antegrade and retrograde drillings of the lesions have been reported, drilling through intact cartilage should be avoided [18]. Some authors further recommend additional bone grafting in cases of large stage 2 lesions [19],[20].

In this paper, the newly developed Fluoro Free navigation procedure for retrograde drilling of lesions under arthroscopic control is evaluated for practicability and precision as compared to the gold standard of fluoroscopy-controlled procedures.

Material and methods

Experimental set-up

Two different femoral models were used: A Sawbone third-generation composite model reflecting normal bone and a Sawbone cortical shell representing an osteoporotic bone (Sawbones Europe, Malmö, Sweden).

Markings representing the locations of the lesions were made on the cortical surface of each condyle in the weight-bearing zones using a pen. Four markings per bone were made at different locations to eliminate any training effect (Figure 1A). Each bone was then placed in a prefabricated soft tissue envelope and securely positioned in an arthroscopic leg holder on a standard operating table (Maquet, Rastatt, Germany) (Figures 1B and 1D).

Figure 1. Operating room set-up. A: Markings on the distal femur representing osteochondral lesions. B: Arthroscopy is used to determine the locations of the markings with a (navigated) pointer. C and D: Set-up for arthroscopically assisted retrograde drilling controlled by 2D fluoroscopy (group 1) or the Fluoro Free navigation technique (group 2).

Operation procedure

A standard knee arthroscopy was performed by introducing the camera and instruments via the standard paramedian lateral and medial portals. The drawn marking was identified and marked with the tip of a pointer under visual arthroscopic control (Figures 1B and 1C). Drilling was controlled as described below for both groups, with the surgeon blinded to the anatomy.

Drillings were directed towards the selected lesion. The target point was virtually set 5 mm subcortical to the marker in line with the starting point of the drilling. This should reflect the center of the OCL and preclude any direct feedback to the surgeon in the form of changing resistance due to different bone densities (cancellous versus cortical) during the drilling. We used a drill sleeve, a 3.2-mm drill bit, and an air-driven drill (Compact Air Drive, Synthes, Bochum, Germany).

For each group, 12 retrograde drillings were performed by the same surgeon (F.G.) to reduce inter-observer variability. The order of drillings was randomized to eliminate any training effect.

Group I: 2D fluoroscopy-controlled technique

The entry point for retrograde drilling was determined by 2D fluoroscopic images (Vario, Ziehm Imaging, Nuremberg, Germany). The sleeve, including the 3.2-mm drill bit, was inserted via a stab incision and blunt preparation to the bone. The retrograde drilling was controlled by two 2D fluoroscopic images acquired at 90° to one another. The final position of the drill tip (a 5-mm offset of the pointer in the direction of the starting point) was controlled solely by fluoroscopy (Figure 2).

Figure 2. Postoperative fluoroscopic control images are acquired after retrograde drillings in both groups.

Group II: Fluoro Free navigation technique

For the Fluoro Free technique, we used an optoelectronic navigation system (VectorVision®, Trauma Software 2.6.1, BrainLAB, Heimstetten, Germany). The system facilitates real-time tracking of different instruments and visualization of their movements in up to four 2D projections, displayed on a sterile covered touch-screen (Figure 3). Tracking is accomplished by means of reflective passive markers attached to each instrument.

Figure 3. The sterile touch-screen of the navigation system displays navigated instruments in three different projections. The labeled items are (a) the sleeve; (b) the drill bit, including the offset (red) and the potential deviation zone at the tip of the drill bit (green cone); (c) the pointer (green) with offset (red); and (d) the virtual center of the osteochondral lesion.

Due to manufacturing restrictions in the most recently available version of the software, a minimum of two X-ray images had to be acquired to determine the space for navigation and to start the navigation procedure. To overcome this problem, we placed the C-arm at the region of interest (where the knee would be positioned afterwards) and acquired three images without bone (antero-posterior, lateral and oblique) before placing the knee in the arthroscopic leg holder on the standard operating table.

The navigated retrograde drilling was commenced after validation of the sleeve and calibration of the drill bit. We navigated both instruments: the sleeve to obtain a precise drilling direction independent of potential drill-bit deviation [5]; and the drill bit to determine the depth of drilling, including the potential maximal deviation (Figure 3). The insertion point at the skin was determined by using a maximum offset (virtual extension) for the drill bit and sleeve. This enables visualization of the planned drilling trajectory and exact positioning of instruments before starting the procedure.

The sleeve including the 3.2-mm drill bit was inserted via a stab incision and blunt preparation to the bone. The alignment and orientation of the drill in relation to the pointer tip including the 5-mm offset (representing the drilling target) was exclusively controlled in the three projections displayed on the navigation screen (Figure 3). The drilling procedure was virtually controlled and visualized in real time on the navigation screen.

Intra- and postoperative evaluation

The duration of the procedure, radiation exposure time and number of corrected drilling maneuvers were recorded for each drilling. Corrections of drilling were defined as retractions necessary to enable a new orientation of the drill direction.

At the end of each procedure, fluoroscopic control images (Figure 2) and CT scans (Lightspeed VCT; GE Medical Systems; slice thickness: 0.625 mm) were acquired for both groups, but only for postoperative precision analysis (Figure 4). To enable visualization of the drawn markings in the CT scan, 0.5-mm metal balls were placed on them (Figure 4).

Figure 4. Image processing and vector calculation was performed based on postoperative CT scans in the three planes of the drilling axis in the femoral condyles. In the center panel, the star indicates a metal ball applied to enable visualization of the marked osteochondral lesion in the CT scans.

Raw data from the CT scan were collected and transferred as DICOM images to specialized analysis software (OsiriX Medical Imaging Software [21],[22]). Coordinates for three points (the drilling start point, the drill end point and the target point) were determined and vector calculation was performed. The distance from the tip of the canal to the lesion was calculated as well as the deviation of the drill.

Statistical analysis

Results are expressed as mean ± standard error. For statistical analysis we used SPSS Version 14.0 (SPSS Inc., Chicago, IL) and the paired Student t-test. P-values less than 0.05 were considered as significant.

Results

We used two different synthetic models (3 Sawbone third-generation femoral models and 3 cortical shells in each group) to simulate the healthy as well as the osteoporotic bone condition. No differences between these models were observed during the retrograde drilling procedures or in the postoperative analyses. For this reason, we pooled the data for both models.

In the navigated group, mean operation time was significantly reduced to 124 ± 12.4 seconds, as compared to 170 ± 12.0 seconds (p = 0.016) for the control group. This was due to a 100% first-pass accuracy of drilling in the navigated group; in the fluoroscopic control group, a mean of 2.5 correction maneuvers per drilling were necessary, including repeated changes of the C-arm position. A high degree of precision was achieved with both methods (a mean distance from drill tip to target point of 2.87 ± 0.32 for group I versus 2.15 ± 0.38 mm for group II, and a mean axis deviation of 3.16 ± 0.35 for group I versus 2.50 ± 0.43 mm for group II). No significant differences were observed for these parameters. No intra-articular penetration of the drill bit was observed in either group. Mean radiation exposure per drilling was 17.8 ± 1.3 seconds under fluoroscopic control, whereas no radiation was necessary when using the navigated technique (p < 0.001).

All 24 procedures were performed without any technical problems.

Discussion

The precise placement of different instruments and implants under fluoroscopic control is an essential step in most orthopaedic surgical procedures. In the case of a potential osteochondral lesion, arthroscopy is a recommended diagnostic procedure to precisely determine the integrity of the cartilage [23]. Depending on the specific stage of the lesion, core decompression of the necrosis is an established and effective therapy option [7] to induce revascularization of this area, anatomically representing a terminal vascular bed. Using this approach, good results have been reported in up to 70% of cases [27],[29]. For a precise core decompression, visual identification of the lesion is mandatory. Antegrade drillings can be controlled by arthroscopy and retrograde drillings by fluoroscopy [18],[30]. Retrograde drilling is the favored technique, since it avoids additional trauma to the intact cartilage surface, but it is also more demanding, resulting in a higher rate of misplacement of the drilling canal. This can lead to uncompressed lesions, intra-articular drill penetrations, and iatrogenic fractures, especially after repeated attempts to reach the necrosis center [7–9],[31].

In addition, this approach is accompanied by radiation exposure for both the patient and surgeon. To reduce this exposure and increase the precision, new techniques for controlling retrograde drilling have been reported. Fink et al. [32] described a navigation procedure based on preoperatively acquired images with an extensive registration procedure. An abbreviated workflow can be achieved by intraoperative acquisition of CT images, but this requires either the presence of a CT scanner in the operating room or sterile conditions in the CT room, neither of which options are available at the majority of institutions. Ohnsorge et al. reported decreased radiation exposure time (29 versus 11 seconds) and higher precision of retrograde drilling (distance from the drill tip to the center of a talus lesion = 2.38 versus 5.43 mm) when using a 2D fluoroscopic navigation system as compared to the standard fluoroscopy-based technique in a cadaver model [31], and similar results have been reported for femoral head lesions [33]. The precision in our study is close to the range of the navigation system itself, for which approximately 1 mm of tolerance has been calculated. Under clinical conditions, using a standard navigation system based on fluoroscopic images, a further benefit can probably be obtained by better visualization of the lesion in images acquired from a fluoroscopic 3D scan [34]. Citak et al. reported a navigation procedure based on intraoperative matching of preoperatively acquired MRI and intraoperatively acquired 3D fluoroscopic images to improve the visualization of an OCL [35]. This is an attractive but so far quite complicated and time-consuming procedure, and is not yet commercial available.

As is common to all navigation systems, rigid fixation of a dynamic reference base (DRB) on patient landmarks is necessary to determine the position of anatomic structures in relation to the navigated instruments, and for all image-based navigation systems (CT, 2D or 3D fluoroscopy) an additional radiation exposure for the patient and operating team is unavoidable. In conjunction with difficult navigation algorithms (image transfer, matching procedure, verification and calibration), this easily leads to prolonged operating time and includes a variety of potential sources of failure. Together, these various drawbacks have resulted in only limited acceptance of navigation systems by many surgeons and operating teams, irrespective of the obvious advantages for different indications [4],[31],[36],[37].

To overcome these limitations, we have developed a Fluoro Free navigation procedure which is adaptable to different operating procedures. This procedure (i) is equally as precise as the gold standard, i.e., fluoroscopically guided drilling; (ii) has a 100% first-pass accuracy; (iii) involves no radiation exposure; (iv) requires no additional fixation of a DRB; and (v) does not extend operating time in cases of arthroscopically controlled retrograde core decompression.

The current limitations are the high cost of purchasing and maintaining a navigation system [34],[38] and the lack of a software module adapted for this new approach. It should be noted that the above-described acquisition of “empty” X-rays (without an anatomic object) for the Fluoro Free navigation technique is only necessary to start the navigation procedure in the recent version of the software algorithm, and will no longer be required once the software is fully perfected.

In addition to this modification, further simplification of the workflow is desirable, including an adapted display for better visualization of the distance between instruments. In this regard, control of axis deviation of the drill bit was not as easy as control of the depth of drilling. This is a well-known phenomenon, and axis deviation correlates with increasing depth of drilling and reduced diameter of the drill [5],[8]. By improving the visualization of instruments on the navigation screen, especially a higher tip distance resolution of instruments and an additional autopilot display, as found in other software modules, a further increase in precision can be expected.

These modifications of the radiation-free navigation technique are currently under development in cooperation with industry and may be integrated as additional navigation tools in future Trauma Software updates. Our findings represent in vitro conditions and therefore cannot be extrapolated directly to in vivo conditions. Further cadaver studies with the new software are necessary before the first clinical applications can be performed.

In conclusion, the developed Fluoro Free navigation procedure is a promising and simplified approach to navigating different instruments as well as implants in relation to visually or tactilely placed pointers or objects without the need for radiation exposure or invasive fixation of a dynamic reference base in the bone.