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

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

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

#### **Figure 3.**

*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).*

*Essentials in Hip and Ankle*

illustrated in **Figure 1**.

**Figure 1.**

**2. Identification of impingement points**

*Flow of computer-assisted hip arthroscopy for FAI.*

**54**

**Figure 2.**

*model and axial image (arrow).*

*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* 

mechanical bone strength after osteochondroplasty using finite element analysis. These postoperative evaluations provide important feedback for future cases, as

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 4.**

*Improvement in the internal rotation angle after each type of resection. The mean improvement in the internal rotation angle in the DDH with cam group after slight resection was significantly greater than that in the DDH without cam group (P = 0.046), as was that after sufficient resection (P = 0.002). The improvement in the cam-FAI group was significantly greater than that in the DDH without cam group (P = 0.043). DDH: developmental dysplasia of the hip.*

cam morphology [6], demonstrating that the improvement in the range of motion is dependent on the extent of bone resection. An interesting clinical implication in this study is that borderline dysplasia with cam morphology showed the most significant improvement by virtual osteochondroplasty. Therefore, even in cases of borderline dysplasia, the co-existence of cam morphology should be considered.
