**2. Identification of impingement points**

The distribution of impingement points in cam lesions is dependent on the morphology in each femoral head-neck junction, as well as acetabular morphology, such as dysplasia [2]. Bone-to-bone impingement occurs during the dynamic phase of hip motion, such as flexion and internal rotation. Therefore, it is not possible to identify the impingement point by radiograph only. Computer modeling based on CT data allows the identification of the exact impingement point in each lower limb position. As a representative position for anterior impingement, maximum internal rotation at a 90° flexion is preferred for impingement simulation (**Figure 2**). The ZedHip (Lexi, Tokyo) system is used for the computer simulation analysis. It is important to note that the impingement point is dependent on the hip position setting, i.e., flexion angle or additional adduction of the hip. The position setting is

#### **Figure 2.**

*Identification of the impingement point by computer simulation. Impingement simulation with 90° flexion and maximum internal rotation is conducted. The impingement point is identified in the three-dimensional femur model and axial image (arrow).*

**55**

**Figure 3.**

*The Practice of Computer-Assisted Planning and Navigation for Hip Arthroscopy*

variable, and it is difficult to define the optimal setting, which is one of the limita-

In a previous study by Oishi et al. [3], impingement simulation was applied and compared with the abnormal uptake point by positron emission tomography. This study showed that impingement may occur more frequently at <90° flexion in patients with FAI syndrome with cam morphology. In addition, the range of hip motion in daily life is activity-dependent. Therefore, further evaluation is required to establish the appropriate hip position setting for impingement simulation representing the variety of hip positions during different activities associated with

**3. Virtual osteochondroplasty and assessment of improved range** 

to the trade-off between bone strength and the risk of residual deformity.

*Virtual osteochondroplasty in preoperative planning. With the central focus on the impingement point, osteochondroplasty is performed in the computer simulation model. The purple colored area shows the postresected bone region; bone resection must be conducted smoothly to avoid creating a notch (arrows).*

After deleting the impinging bony region during virtual osteochondroplasty, a bone model is reconstructed. Using this model, the range of motion can be re-evaluated and compared with the situation prior to virtual osteochondroplasty. It is difficult to judge the improvement in maximum internal rotation angle; however, we suggest that the threshold is set at 10°. Therefore, virtual osteochondroplasty should be performed until an improvement of more than 10° is confirmed. Kubota et al. evaluated the improvement of range of motion after virtual osteochondroplasty, comparing FAI and borderline dysplasia cases [6]. **Figure 4** shows the difference in improvement between cam-type FAI and borderline dysplasia with or without

The next step in the preoperative planning process is computerized "virtual osteochondroplasty" (**Figure 3**). The bony region is deleted in the editing mode of the ZedHip software with the central focus on the impingement point. It is important to delete the bony region smoothly, rather than creating a notch, as it is with the actual osteochondroplasty. Flexion angle at 70 or 45° should be added as a simulation condition. It should be noted that virtual osteochondroplasty should be performed using the same assumptions as actual surgery, i.e., excessive osteochondroplasty should be avoided to maintain bone strength. A previous study using an animal model of mechanical testing and finite element analysis revealed that up to 36% of the femoral neck diameter could be safely resected during simulated osteochondroplasty [4]. By contrast, residual cam deformity is one of the most important risk factors for revision surgery [5]. It is, therefore, important to pay close attention

*DOI: http://dx.doi.org/10.5772/intechopen.89502*

daily life.

**of motion**

tions of this approach, as discussed later in this chapter.

#### *The Practice of Computer-Assisted Planning and Navigation for Hip Arthroscopy DOI: http://dx.doi.org/10.5772/intechopen.89502*

variable, and it is difficult to define the optimal setting, which is one of the limitations of this approach, as discussed later in this chapter.

In a previous study by Oishi et al. [3], impingement simulation was applied and compared with the abnormal uptake point by positron emission tomography. This study showed that impingement may occur more frequently at <90° flexion in patients with FAI syndrome with cam morphology. In addition, the range of hip motion in daily life is activity-dependent. Therefore, further evaluation is required to establish the appropriate hip position setting for impingement simulation representing the variety of hip positions during different activities associated with daily life.
