63 Principles of Computer-Aided Surgery in Trauma Surgery

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Principles of Computer-Aided Surgery in Trauma Surgery

Y. Weil, R. Mosheiff, L. Joskowicz, M. Liebergall

Introduction Computer-Aided Surgery (CAS) has started to have a significant impact on the practice of orthopaedic surgery [1]. Together with concurrent surgical and technologi-cal trends, such as minimal invasiveness and improved digital imaging, it has the potential to improve results, shorten operative time, reduce morbidity, and reduce variability in a number of common surgical orthopaedic procedures.Despite a slow beginning, in the last five years CAS has made inroads in orthopaedic trauma by providing sup-port for the planning and execution of surgical proce-dures. Preoperative planning consists, among others, of digital two and three-dimensional implant templating with properly scaled digital X-ray and CT images, accu-rate measurements of anatomical angles and axes, and osteotomy planning,. Intra-operative execution consists of fluoroscopy and CT-based navigation of surgical instru-ments and implants, and in some cases, mechatronics and robotic devices.Navigation systems are by far the most commonly used in orthopaedic trauma and hence our discussion is fo-cussed on them. First, we describe the types of navigation systems currently used in orthopaedic trauma. Next, we discuss the clinical considerations for fluoroscopy-based navigated trauma surgery and describe the clinical proce-dures in which it is used. We conclude with perspectives and our view of future developments.

Navigation Systems for Trauma

There are four types of navigation systems currently in use in orthopaedics: 1) CT-based; 2) two-dimensional (conventional) fluoroscopy-based; 3) three-dimensional (volumetric) fluoroscopy-based; and 4) imageless (kine-matics). All of them rely on optical tracking technology.

CT-based Navigation

Introduced in the early 1990s, CT-based navigation systems were the first-image guidance systems used in orthopae-dics. They were designed to assist surgeons in pedicle screw insertion and in acetabular joint replacement. Preoperative planning consists of determining the location and diameter of the pedicle screw pilot holes; the implant and/or ac-etabular cup type and size; and location on a preoperative CT. Intra-operatively, a dynamic reference frame is rigidly attached on or near the site of interest to a bony structure and the CT images are aligned (registered) to the patient’s anatomy. The plan, together with a graphical template of the surgical tools in their real-time location are then superim-posed on the CT images and shown on a computer screen.

The first CT-based systems in trauma appeared in the late 1990’s for percutaneous iliosacral screw insertion and for intra-medullary nailing of femoral shaft fractures [2]. The two main advantages of CT-based systems over con-ventional fluoroscopy are: 1) they provide axial and spa-tial, real-time multi-image visualization of bony anatomy and surgical tools and 2) they significantly reduce the use of fluoroscopy.

The current use of CT-based systems in orthopaedic trauma is very limited. First, preoperative CT images of long bone fractures are not always indicated and might delay definitive fracture care and increase the burden on the health care system. Second, even when indicated and available (e.g. pelvic fractures), a registration pro-cess, which can be cumbersome, error-prone, and time consuming is required. Third, unstable fractures cause bone displacement that alters the registration, introducing errors that invalidate image-based navigation. Although, useful, most trauma applications have moved away from CT-based navigation.

Intra-operative CT or MRI-based navigation systems overcome some of these drawbacks but are cumbersome, costly, and can lengthen the operative time. Furthermore, their availability is very restricted [3–5].

Two-dimensional Fluoroscopy-Based Navigation

Conventional two dimensional C-arm fluoroscopy-based navigation systems are the most commonly used naviga-tion systems in current orthopaedic surgery [6]. Their main advantage is that they are the closest to conventional fluoroscopy, the »work horse« of orthopaedic trauma. In these systems, a tracking and calibration cage is attached to the C-arm image intensifier and draped for sterility. Once the patient is prepared on the operating table, the surgeon attaches a dynamic reference frame on or near the bony structure of interest and acquires a set of fluo-roscopic images. The images are stored and shown on a computer screen with the actual surgical instruments’ location superimposed on them, in effect creating aug-mented, multiplanar real-time virtual fluoroscopy. Ad-ditional fluoroscopic images can be acquired during the procedure, both for validation and for navigation.

There are many advantages to fluoroscopy-based navi-gation. First, it enables surgeons to better plan and execute surgical actions without the need to perform actual ad-ditional fluoroscopy. For example, surgeons can plan and correct the direction of straight implants such as percuta-neous screws or intra-medullary nails, or plan the extrac-tion of foreign bodies, by simultaneously following the im-plant and surgical instruments’ positions on all augmented fluoroscopic images. This is a significant improvement over the conventional method, in which implants are posi-tioned by repeatedly adjusting the implant location in each plane (typically coronal and sagittal), and acquiring a con-

trol image for each step of the procedure. This results in better implant placements, shorter time, and significantly less radiation to the patient and surgical staff (add ref). Unlike CT-based systems, no intraoperative registration is necessary. However, the line of sight between the position sensor (camera) and the trackers must be maintained at all times, and the dynamic reference frame must remain rig-idly attached throughout the procedure. Unlike CT-based navigation systems, no axial or spatial views are available.

Three-dimensional Fluoroscopy-based Navigation

A recent addition to the orthopaedic intra-operative im-aging arsenal is intra-operative CT-like imaging with an isocentric 3D fluoroscope (e.g. the Siremobil IsoC-3D, Sie-mens, Germany). This new type of fluoroscope is a motor-ized C-arm unit that captures up to 100 two-dimensional fluoroscopic images in sequence by rotating the C-arm by almost 190° around the anatomical region of interest. It produces axial CT-like slices with 25–50% less radiation than that required by a conventional high-resolution CT scanner (reference). Despite the lower image quality, the resulting images show sufficient detail of bony anatomy and are adequate for most trauma applications [7, 8].

Navigation with three-dimensional fluoroscopy pro-ceeds as with conventional fluoroscopy. However, the field of interest changes along the course of the surgical instru-ment thus the image is updated according to the precise spatial location of the tool (⊡ Fig. 63.❚). A set of three-di-mensional fluoroscopic images are acquired after C-arm draping and reference frame attachment. The actual axial views and the computed sagittal, coronal, oblique, and spatial images can be viewed simultaneously with the tracked surgical instruments superimposed on them. Ad-ditional two- and three-dimensional fluoroscopic images can be acquired during the procedure.

The advantages of three-dimensional fluoroscopy are that they provide intra-operative CT-like images without additional registration. However, the field of view is cur-rently small (9" in current fluoroscopes, unsuitable for long trajectories), image quality is limited (e.g. in the thoracic region and in obese patients), and radiation dos-age is significant. To date, it has been successfully used for inserting pedicular screws [9] and for retrograde drilling of osteochondral talar lesions in treating foot and ankle lesions [10].

Chapter 63 · Principles of Computer-Aided Surgery in Trauma Surgery63485

Imageless Navigation

A recent addition to computer-aided orthopaedic naviga-tion is imageless navigation. Today, it is predominantly used in total hip and total knee arthroplasties. In image-less navigation, a dynamic reference frame is attached to at least two bone sites (e.g. femur and tibia). The extremi-ties are then moved and the kinematic and joint axis data is acquired. The relative angular data, such as hip inclina-tion and anteversion are updated in real time. For total knee arthroplasty, the surface of the tibial plateau can be sampled with a pointer to reconstruct its shape and plan the cuts.

The advantage of imageless navigation is that it pro-vides kinematic and joint motion data that is difficult or impossible to obtain with images. While this technique is effective in joint reconstruction and can be useful for fracture reduction, no use of it has to date been reported in orthopaedic trauma.

Clinical Considerations for Navigated Fluoroscopy-based Trauma Surgery

Fluoroscopy-based navigation is by far the most com-monly used type of navigation in orthopaedic trauma. Common important issues include indications, hardware and setup, attachment of the reference frame, fluoroscopic image capture, and radiation reduction.

Indications. Fluoroscopy-based navigation relies on pre-viously acquired images. Thus, a constant and stable fracture reduction is a prerequisite for navigation. Non-displaced or provisionally reduced fractures by means of traction, external fixators and Kirschner wires are examples of appropriate fixation.

Hardware and Setup. Fluoroscopy-based navigation re-quires the following equipment: 1) a pre-calibrated cus-tom surgical instrument or a standard one fitted with a tracker rigidly attached to it; 2) a calibration and tracking cage attached to the C-arm image intensifier; 3) a refer-ence frame rigidly attached to the bony anatomy and; 4) a position sensor (usually an optical tracker camera) positioned in the operating room so as to maintain at all times a line of sight with the C-arm, the dynamic refer-ence frame, and the tracked surgical instrument. The ad-

ditional navigation setup time and effort is minimal once the surgeon has understood the navigation requirements and has become familiar with them.

Attachment of the Reference Frame. Fluoroscopy-based navigation requires rigidly attaching a reference frame on or near the anatomy of the operated site to compensate for its motions. The actual attachment loca-tion is determined by anatomical, procedural, and tech-nical requirements. For example, the greater trochanter is the usual site for navigated femoral intra-medullary nailing. When the reference frame is too close to the sur-gical site, possibly interfering with surgical instruments, it should be moved out of the way. However, the further the reference frame is from the tracked instrument, the lower is the tracking accuracy, as angular and transla-tional errors increase. A distance of up to 20 cm between the tracked instrument and the reference frame yields acceptable results in terms of accuracy with current opti-cal tracking technology. In a recent study of cannulated screw fixation of the hip [11], the reference frame was alternatively placed on the ipsilateral iliac crest and on the fracture table, with the patient tightly secured to it, instead of on the greater trochater. The errors were 1.5 mm and 1o in translation and rotation; acceptable for this procedure.

Fluoroscopic Image Capture. The type of views and number of fluoroscopic images to be used in navigation is procedure-dependent. For example, the insertion of a percutaneous iliosacral screw for posterior pelvic ring disruptions requires 2–4 views of the pelvis (inlet, outlet, lateral and roll over) [12]. The percutaneous insertion of cannulated screws for hip fractures requires a single AP and lateral images of the hip.

Radiation Reduction. An advantage of fluoroscopy-based navigation is the reduction of radiation to both the patient and the surgical team. One minute of intra-operative fluoroscopy of the pelvis is equivalent to about 40 mSv of radiation, 250 radiographs of the chest, or one pelvic CT scan [6, 13]. Thus, the maximum allowable amount of radiation to the hands and eyes is reached after 50 cases [13]. A prospective randomized study of navi-gated versus standard distal locking of intra-medullary nails showed significant radiation reduction when using navigation [14].

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486 Part V · Reconstruction and Trauma Surgery

Clinical Applications of Navigated Fluoroscopy-based Trauma Surgery

We now describe the clinical applications of fluoroscopy-based navigation in four types of surgical procedures. The first three support fracture fixation: 1) trajectory naviga-tion with two-dimensional fluoroscopy; 2) navigation with three-dimensional fluoroscopy; and 3) navigation of complex-shaped instruments and implants. The fourth supports fracture reduction of long bone fractures.

Trajectory Navigation with Two-dimensional Fluoroscopy

Insertion of straight surgical fixation implants such as screws, nails, or wires is an everyday task in orthopaedic traumatolgy. Often times, such procedures are performed percutaneously or through small incisions, without actual visualization of the target organ. Usually, fluoroscopic imaging is used to monitor the direction of the implant in the target bone using multiple projection, typically a minimum of two orthogonal images (such as anteropos-terior and lateral). Significant skill, practice, and time are required to perform this hand/eye coordination task involving the mental reconstruction of a dynamic spatial situation from a sequence of static images.

Navigated two-dimensional fluoroscopy provides a natural computerized enhancement for this surgical ac-tion. By simultaneously following the real-time position of both the actual surgical instrument tip and axis, and its projected straight-line trajectory on one or more im-ages, the surgeon can aim and continuously correct the position and orientation of the surgical instrument with respect to a target. This improved surgeon’s hand/eye co-ordination results in more accurate placements in shorter time, with less radiation.

A tracked, pre-calibrated drill guide is used in most applications (⊡ Fig. 63.1). The drill guide is a cannula with interchangeable internal sleeves of various diameters through which drill bits and guide wires are introduced. The computer extrapolates the trajectory of the planned implant and superimposes it on the previously acquired fluoroscopic images. For visualization, the trajectory line diameter and extension can be chosen as appropriate (see ⊡ Fig. 63.1, bottom). To place a screw, the surgeon brings the tracked drill guide tip to the entry point and orients it according to the augmented images until the axis co-incides with the desired target direction. Since these ad-justments are done before the actual drilling takes place, drilling attempts and »false routes« are avoided [15].

Most commercial systems allow for attaching trackers to long straight surgical instruments such as awls, taps, drill bits, and graspers and for calibrating them intra-


⊡ Fig. 63.1. Insertion of a percutaneous ilio-sacral screw using two-dimensional fluoros-copy-based navigation. The pink line shows the actual drill guide axis and tip; the green one its predicted trajectory. The diameter and extension of the trajectory line are shown at the center bottom of the screen.

operatively. This allows for tracking instrument progress into the bone after insertion and to correct its position ac-cordingly. However, the surgeon must watch for bending and deflection of long instruments, since this may result in inaccurate results [15].

It is important to once again emphasize that any actual change in anatomy, such as fracture displacement or motion of the reference frame will cause a mismatch between the displayed image and the trajectory shown on the screen. If any suspicion arises, additional fluoroscopic images should be acquired for confirmation.

We next describe four routine clinical surgical appli-cations for trajectory navigation: 1) percutaneous pelvic ring and acetabular fractures fixation with a cannulated screw; 2) screw removal; 3) hip fracture pinning; and 4) intramedullary nailing.

Percutaneous Pelvic Ring and Acetabular FracturesPercutaneous iliosacral screw insertion is a demanding procedure: the »safe zone« for screw placement is narrow due to the presence of nearby nerve structures [16]. Routt [17] has shown that the screws can be safely inserted by adjusting the drill bit trajectory based on alternate pelvic inlet and outlet fluoroscopic images. While CT-based navigation has also been used to place the screws, the most practical method is navigated two-dimensional fluoroscopy. The desired trajectory is planned based on a pair of pelvic inlet and outlet views, and the drill guide trajectory is adjusted to be on the center of the safe zone in all images. A single drill attempt is performed, with a very high probability of being inside the safe zone. Opera-tive time is thus reduced to a few minutes per screw, and radiation dosage is similarly reduced [12, 18]. Note, that multiple screws can be placed using the same images, and therefore stronger constructs can be installed to increase safety [19].

Other screws for the fixation of pelvic and acetabular disruptions can be placed percutaneously in a similar fashion, and these in turn may assist in minimizing the surgical dissection associated with formal approaches. A classical example is the placement of an anterior column screw for the transverse component of an acetabular frac-ture via a posterior approach. While these screws are not placed percutaneously, their placement requires repeated use of intra-operative fluoroscopy, which can be totally avoided with navigation [12, 20].

Screw and Hardware RemovalThe removal of pelvic fixation screws and other hardware is indicated in some cases, such as for women of child bearing age. This task may be challenging since these implants are deeply seated and extensive soft tissue dissec-tion cannot be avoided. Fluoroscopy-based navigation can be used to direct the surgeon towards the implants, thus reducing the risk of damage to soft tissues and decreasing the radiation to the pelvis [21]. The technique is identical to that of pelvic screw insertion, with the difference being that the surgeon reaches for the head of the screw with the navigated screwdriver. Other applications include the removal of broken screws, missiles or other foreign bodies with pre-calibrated special drills and instruments, such as a hollow mill or a cement grabber.

Hip Fracture PinningCannulated screw fixation of hip fractures is a com-mon undertaking with conventional fluoroscopy. Recent studies [22–25] show that particular screw spreads and configurations are preferred in order to prevent compli-cations such as collapse and subtrochanteric fractures. Navigation can be used to achieve the desired spread. A study comparing the accuracy of navigated cannulated screw insertion with fluoroscopy-based navigation versus conventional fluoroscopy, demonstrated superior forma-tion of the screws with respect to parallelism and spread, with fewer overall complications [26]. Other studies have also demonstrated increased accuracy in the pinning of slipped capital femoral epiphysis in adolescents [27, 28].

The surgical technique for placing these screws is similar to the one presented above. First, the fracture is re-duced and the reference frame is attached to the involved side iliac crest. Next, a pair of AP and lateral images of the involved hip are acquired and used for insertion of all three screws. A verification X-ray image is shot after all guide wires are in place and screws are inserted in the standard fashion.

Intra-medullary NailingAlthough routinely performed, intra-medullary nailing is not devoid of complications. Two issues are starting point selection and screw locking. Accurately placing the starting point can prevent complications such as avascular necrosis of the femoral head, unnecessary cartilage dam-age (especially in the knee and shoulder), fractures, and

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risk to neurovascular structures. Locking screw insertion requires repeated trial and error, usually with heavy use of fluoroscopy.

Trajectory navigation with fluoroscopy is indicated for both starting point selection and screw locking [29]. The starting point can be determined with a tracked

cannulated drill guide by choosing an entry point and ob-serving the planned trajectory on the screen (⊡ Fig. 63.2). Once the fracture has been reduced and the nail inserted, freehand navigated screw locking can be performed with the »perfect circle« technique (⊡ Fig. 63.3). First, the refer-ence frame is fixed to the bony anatomy near the locking


⊡ Fig. 63.2. Computerized assisted nailing of a tibial shaft fracture: determination of the entry point.

⊡ Fig. 63.3. Computer screen showing lateral view of distal locking. The concentric circles show the position and orientation of the drill.

screws’ location. Then, an AP view in which the nail holes appear as circles is acquired. An additional lateral view may be taken to determine screw length measurement. The tracked drill guide is then drawn towards the lock-ing screw area and is navigated until a circle within the hole appears on the computer screen. This is followed by drilling through the tracked drill guide and inserting the locking screw. Sometimes, such as in the case of the tibial nail, the same AP and lateral views can be used for inser-tion of two or three adjacent locking screws. A single drill attempt usually suffices for the locking screw. In a similar fashion, a blocking or »poller« screw for stabilizing nar-row nails in a wide metaphyseal bone can be planned and inserted.

Navigation with Three-dimensional Fluoroscopy

Three-dimensional fluoroscopy by itself is useful in the reduction of articular fractures such as calcaneal, tibial plateau or plafond, acetabular, distal radius, etc. It may provide data regarding articular surface congruency, im-plant position and intra-articular fragments or hardware and may change the course of a case [10, 30].

Coupled with navigation, three-dimensional fluoros-copy provides the main advantage of CT-based naviga-

tion; axial and spatial views, without the need for registra-tion. Although indications for the use of this modality are currently evolving, they include pedicle screw insertion [18] and percutaneous fixation of non-displaced or pro-visionally reduced fractures in which the field of view is relatively narrow (e.g., acetabular, calcaneal, talar, proxi-mal and distal tibia). A preliminary report of three-di-mensional fluoroscopic navigation in talar osteochondral lesions is promising [31]. The main benefit is the recogni-tion of occult fractures and the direction of the implant in the optimal direction for fixation (⊡ Fig. 63.4), which may be unachievable with conventional fluoroscopy.

The current limitations of navigated three-dimen-sional fluoroscopy are the narrow field of view and the radiation dosage.

Navigation with Complex-shaped Instruments and Implants

One of the limitations of commercial fluoroscopy-based navigation systems is that they only allow for the tracking of straight instruments. This excludes orthopaedic surgi-cal hardware such as plates and most nails. Specific ap-plications, notably in total hip arthroplasty, allow for the navigation of rasps and cups by showing the two dimen-

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⊡ Fig. 63.4. An acetabular fracture treated by navigated percutaneous fixation using three-dimensional fluoroscopy.

sional projection of both cup and stem in order to assess component placement, especially in the axial plane [32].

Navigating complex-shaped, non-straight instruments and implants in orthopaedic trauma is still in its infancy. Two examples include including a preliminary trial with a LISS plate [33] and a design for the Gottfried PCCP plate (⊡ Fig. 63.5). To navigate these instruments, a precise description of their geometry must be made available to the navigation system, along with their custom calibration specifications. The navigation system then computes the silhouette of the instruments and superimposes them on the fluoroscopic images. The main advantage is that the actual implant profile and position are simultaneously shown in the fluoroscopic images, facilitating its final posi-tioning and thereby saving operative and radiation time.

Navigation in Fracture Reduction

Long bone fracture reduction, especially of the femoral shaft, is considered to be the key step in the treatment of skeletal injuries. Currently, the reduction requires exten-sive use of fluoroscopy, with inevitable exposure to the surgeon’s hands. Inaccurate results, especially in peri-axial rotation, are not uncommon and may result in long term complications [34, 35].

Fluoroscopy-based navigation for long bone fracture reduction is obviously appealing, although to date only a couple of commercial system support it. The first obstacle is technical, as earlier systems did not support tracking of two bony segments with a reference frame attached to each. Traditionally, the navigation systems track only a surgical instrument and a reference frame. An elegant solution is to insert a tracked intra-medullary device in one bone fragment and tracking the other fragment with the bone tracker for fracture reduction [36]. Systems that allow tracking and alignment of two fragments are under development (⊡ Fig. 63.6).

Perspectives and Conclusions

Wider availability of navigation systems and their routine clinical use, along with support for more procedures and instruments are the most immediate issues that will contribute to the advancement of CAS in orthopaedic trauma.

In addition, computer-aided navigation can support surgery planning in the operating room, shortly before or even during surgery. The navigation system can be used as an intra-operative measuring device, thus providing valu-able data that blurs the distinction between preoperative


⊡ Fig. 63.5. Navigation of a complex-shaped implant – and experimental model of the Gotfried PerCutaneous Compression Plate (PCCP).

and intra-operative planning. Examples of such planning include:1. performing accurate measurements on the post re-

duction fracture for the assessment of restoration of normal anatomy thus helping in the planning of the reduction;

2. determining an implant position after fracture re-duction by positioning an implant template on post reduction fluoroscopic images;

3. planning a fracture reduction by identifying the axes of each fragment in multiple planes or by graphically outlining the fracture fragments on the fluoroscopic images and then aligning them;

4. planning the trajectory and length of a fixation device, such as a screw or an intramedullary nail, thereby avoiding inappropriate or misplaced hardware; and

5. planning the restoration of normal limb alignment, such as planning the poller (blocking) screw position in intra-medullary nailing of metaphyseal fractures. To avoid coronal or sagittal deviation of the limb axis due to narrow nail diameter, a metaphyseal screw can be implanted outside the nail to reduce the volume available for the nail and to assist in fracture reduction.

Fracture reduction is a topic that elicits much interest and for which planning and navigation modules are being de-veloped. When available, the anatomy of the healthy side can be used as a guide for the reduction. This compara-tive method has been successfully used with CT-based systems in total knee and total hip arthroplasty. CT-based planning enables the matching of both surface anatomy and peri-axial rotation based on the relation of the femo-ral neck anteversion to the posterior condylar line with respect to the healthy side [37].

In trauma, a fluoroscopy-based method is more desir-able. The main issues are: 1) the definition of fragment boundaries; 2) coronal, sagittal, and axial alignment; and 3) ease of use of the system.

Together with other trends, such as minimal invasive-ness, CAS also has the potential to bring about a paradigm shift in the treatment of trauma. Besides its current main use in intra-operative planning and support, CAS tech-nology serves as the basis for basic and clinical research, quality control, and surgeon training.

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63 Principles of Computer-Aided Surgery in Trauma Surgery

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