Computer-Assisted Surgery for Femoroacetabular Impingement

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CHAPTER 11 Computer-Assisted Surgery for Femoroacetabular Impingement

Asheesh Bedi, Jérome Tonetti, Laurence Chabanas, Stéphane Lavallée, Bryan T. Kelly

Femoroacetabular impingement (FAI) has recently been identified as the predominant cause of labral tears in the nondysplastic hip. Structural abnormalities in the morphology of the hip can limit motion and result in repetitive impact of the proximal femoral neck against the acetabular labrum and its adjacent cartilage. Bony impingement can result from a decrease in femoral head-neck offset (cam effect), an overgrowth of the bony acetabulum (pincer effect), excessive acetabular retroversion, or a combination of these deformities. Bony impingement with range of motion, particularly with internal rotation and flexion, can compromise the labrum and adjacent soft tissues, ultimately resulting in irreversible damage to the articular cartilage and early-onset joint degeneration. These recognized changes in the hip caused by impingement have all been associated with early-onset osteoarthritis of the young hip that was previously thought to be of idiopathic origin.

Arthroscopic treatment of FAI has grown in popularity during the last decade. Advantages include minimally invasive access to the hip joint, peripheral compartments, and associated soft tissues. Furthermore, arthroscopy allows for a dynamic intraoperative assessment and correction of the offending lesions. Although initially met with skepticism, the early results of arthroscopic treatment of FAI have shown favorable outcomes. A recent systematic review reported 67% to 100% good to excellent short-term clinical outcomes after arthroscopic treatment of hip impingement.¹ The long-term viability of these outcomes, however, is predicated on meticulous intraoperative evaluation and a thorough and accurate correction of impingement lesions on both the femoral and acetabular sides. Failure of arthroscopic techniques for FAI is most commonly associated with incomplete decompression of the associated bony anatomy.²

In this chapter, we first review the requirements for an ideal computer-assisted surgery (CAS) system dedicated to FAI by considering the major clinical issues that are encountered and also define our expectations for each of those issues. In the second section, we present an overview of the state-of-the-art systems and prototypes available, which indicate promising preliminary results that need to be further developed.

REQUIREMENTS FOR A COMPUTER-ASSISTED SYSTEM

A computer-assisted surgery system dedicated to FAI could provide several advantages for the diagnosis and management of this disorder. Appropriate treatment of all sources of mechanical impingement, together with refixation of any viable labral tissue, is the recommended treatment intervention in symptomatic young adults to optimize the chances of a successful outcome and return to sport. The challenge remains, however, to diagnose, visualize, and eliminate the sources of impingement reliably, which can often be subtle and are only manifest with the use of provocative dynamic maneuvers.

Preoperative Considerations

Cam impingement is the result of a loss of the normal sphericity of the femoral head from congenital, developmental, or post-traumatic changes in the shape of the proximal femur. The deformity typically occurs at the anterolateral aspect of the junction between the femoral head and neck, but can occur in any location around the circumference of the head-neck junction. Cam impingement results in a characteristic injury pattern to the transition zone cartilage of the acetabulum, where the labrum loses its structural attachment to the adjacent hyaline cartilage.

Pincer impingement results from overcoverage of the acetabulum, resulting in a specific pattern of labral degeneration as the overhanging bone on the acetabulum crushes the labrum during movement; this produces one or more cleavage planes of variable depth within the substance of the labrum. Subtypes of pincer morphology have been identified—focal anterosuperior overcoverage, coxa profunda, acetabular protrusio, and acetabular retroversion.³ Identification of the specific type and location of pincer pathology is critical to developing an appropriate treatment plan that will reliably eliminate mechanical impingement postoperatively.

In addition to cam and pincer lesions, other geometric abnormalities can contribute to symptomatic hip pain and/or mechanical impingement. Recognizing these concomitant lesions is of tantamount importance to guide an appropriate surgical intervention. Femoral retroversion or excessive anteversion must be recognized. Coxa valga can also alter contact mechanics and kinematics of the hip. Acetabular dysplasia, including lateral and/or anterior undercoverage, is also not uncommon and can dramatically affect the load distribution and kinematics of the hip joint. It is essential to recognize the presence of these concomitant factors to develop an effective treatment plan for FAI.

In light of this geometric complexity, a CAS system could prove invaluable in the preoperative diagnostic and planning phases of FAI. The system could assimilate all the geometric considerations noted into a three-dimensional model. More importantly, however, such a system could subsequently subject the model to a dynamic mobility simulation to define any mechanical sources of impingement accurately. These simulations could be individualized based on patient activity level, ranging from simple gait to complex dance maneuvers or pivots required of competitive athletes. A virtual resection of varying degrees on both the femoral and acetabular sides could be performed and the dynamic analysis used to define the minimal resection on the femoral and acetabular sides that would be necessary to relieve mechanical impingement without adversely affecting the stability or kinematics of the joint. With these locations defined, this system could further indicate the ideal portals and trajectory to access these lesions while minimizing trauma to the periarticular soft tissues.

Intraoperative Considerations

There are a number of technical issues associated with arthroscopic treatment of FAI that could substantially benefit from a computer assistance system. These include but are not limited to the following factors.

Portals and Access

Currently, relatively universal portals are used to gain access into the intra-articular and peripheral compartments of the hip. Although these have been effective in treating cam and pincer lesions in the most typical locations, they can be less than optimal for more complex or large lesions. In particular, cam lesions that extend posteriorly and pincer lesions that extend far posteriorly or medially can be difficult to access safely with standard portals without placing neurovascular structures and the vascular supply to the femoral head at risk. Computer navigation systems could provide individualized novel portals to access these lesions safely and directly when necessary. Furthermore, trauma to the abductors and periarticular muscular envelope can be minimized by defining portals that use intermuscular planes and offer the shortest, most direct trajectory to the impingement lesions.

Another important consideration is exposure of the peripheral compartment. Current techniques use a moderate to large capsulotomy or capsulectomy to visualize the femoral neck adequately and define the margins of the cam lesion. This exposure is necessary with current techniques because successful treatment is predicated on thorough visualization and resection of the sources of impingement. However, such extensile exposure may not be without morbidity. Postoperative instability and hip dislocation have been reported after arthroscopic treatment of FAI. Furthermore, attempts at capsular closure after resection are technically very difficult and can be time-consuming, assuming sufficient tissue has been preserved. A computer assistance system may obviate the need and morbidity associated with such extensile exposures because the lesion can be precisely localized and resected through minimal capsular defects.

Localization and Resection of Impingement Lesions

One of the most difficult aspects of the arthroscopic management of FAI remains localizing and defining the margins of the cam and pincer lesions. Even in the setting of an adequate exposure, the margins of the cam and pincer lesion are often not readily apparent. Experienced hip arthroscopists acknowledge that correlation of the location on preoperative imaging studies with intraoperative anatomy can be difficult, particularly with subtle and/or broad lesions that lack clearly discernible margins. Atypical cam lesions that extend posteriorly may not be immediately apparent and may require extensive internal rotation and accessory portals to perform an adequate resection.

Even if the locations of the cam and/or pincer lesions are accurately defined, the magnitude of resection that should be performed is perhaps even more difficult to define. Unfortunately, current techniques rely on the surgeon’s judgment to define an adequate resection that will relieve mechanical impingement without compromising normal structural anatomy. Less experienced hip arthroscopists will tend to perform a less aggressive resection to avoid the risk of a femoral neck fracture or iatrogenic acetabular dysplasia from an overzealous resection. Unfortunately, this tendency leads to a high risk for residual impingement and unfavorable long-term outcomes. On the other hand, aggressive resections that extend beyond the margins of the cam lesion can compromise the cortical support of the native femoral neck and precipitate femoral neck fracture, a significant and potentially devastating postoperative complication. In addition, errant overresection of a pincer lesion on the acetabular side can result in iatrogenic undercoverage and dysplasia.

A computer assistance system could address both these technical issues reliably in the hands of adept and inexperienced hip arthroscopists. The system could merge preoperative imaging findings with real-time intraoperative anatomy, helping the surgeon to define the margins of the lesions unequivocally. Furthermore, the system could provide feedback to guide a complete resection to the appropriate depth while avoiding risk to native structures that do not contribute to the mechanical impingement. Such a system would decrease the dependence of intraoperative surgical judgment to define an adequate and precise resection of all pathology arbitrarily, and allow favorable outcomes to be reliably achieved, even by relatively inexperienced surgeons.

Fluoroscopy

Current techniques in hip arthroscopy rely extensively on intraoperative fluoroscopy throughout the procedure. It is used to guide portal placement and access into the intra-articular compartment, and is also a “poor man’s” guide to localize cam and pincer lesions and confirm adequate resection. Unfortunately, this modality is certainly operator-dependent and limited by the quality of the images that can be obtained intraoperatively, given the limitations of patient positioning. Furthermore, under the best of circumstances, fluoroscopy provides two-dimensional feedback for three-dimensional pathology and can provide limited information regarding the adequacy of resection in multiple planes. In addition, the extensive use of fluoroscopy intraoperatively certainly exposes the patient and surgeon to radiation that can be substantial over the course of a surgical career.

A computer assistance system may significantly reduce the need for intraoperative fluoroscopy and reduce the risks associated with its use. The feedback provided by the system could guide safe and direct portal placement and allow the surgeon to perform an adequate and thorough resection of the cam and/or pincer lesions based on more accurate, three-dimensional data that are obtained preoperatively.

Dynamic Assessment

Perhaps one of the greatest advantages of a computer assistance system is its ability to provide immediate feedback regarding the precise and accurate treatment of FAI via dynamic simulation. FAI is a dynamic pathology that manifests itself symptomatically during provocative maneuvers of the hip that are unique to individual patient activities. Current arthroscopic techniques do not allow for a complete dynamic evaluation of the recontoured hip because of constraints of patient positioning, instrumentation, and swelling caused by fluid extravasation during the procedure. A computer assistance system could assess the kinematics of the hip intraoperatively to confirm that the performed resection has eliminated mechanical impingement with previously symptomatic maneuvers.

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