Orthopedics

Case Report 

Using Imageless Optical Navigation to Identify the New Hip Center in Crowe IV Dysplasia

Colin Yi-Loong Woon, MD; Jeffrey M. Muir, MSc, DC, MSc (Clin Epi); Edwin P. Su, MD

Abstract

In total hip arthroplasty for patients with acetabular dysplasia, cup placement in the native acetabulum is preferred to placement in the pseudoacetabulum. Identifying the true acetabulum may prove challenging. In a patient with Crowe IV dysplasia, 3-dimensional mini-optical navigation was used to match the new hip center to the preoperative radiographic plan, which was identified to be 34 mm inferior to the pseudoacetabulum. This allowed titration of femoral shortening to 20 mm, to arrive at final limb lengthening of 14 mm. Although the use of other enabling technologies in hip dysplasia has been reported, to the authors' knowledge, this is the first reported case demonstrating the use of imageless optical navigation in this setting. It is a navigational tool with a small spatial footprint, does not mandate preoperative axial studies, and does not require multipoint bone surface registration. Imageless navigation may be a useful option for cup positioning and subsequent titration of femoral shortening in the reconstruction of Crowe IV dysplastic hips with degenerative joint disease. [Orthopedics. 2020; 43(2):e119–e122.]

Abstract

In total hip arthroplasty for patients with acetabular dysplasia, cup placement in the native acetabulum is preferred to placement in the pseudoacetabulum. Identifying the true acetabulum may prove challenging. In a patient with Crowe IV dysplasia, 3-dimensional mini-optical navigation was used to match the new hip center to the preoperative radiographic plan, which was identified to be 34 mm inferior to the pseudoacetabulum. This allowed titration of femoral shortening to 20 mm, to arrive at final limb lengthening of 14 mm. Although the use of other enabling technologies in hip dysplasia has been reported, to the authors' knowledge, this is the first reported case demonstrating the use of imageless optical navigation in this setting. It is a navigational tool with a small spatial footprint, does not mandate preoperative axial studies, and does not require multipoint bone surface registration. Imageless navigation may be a useful option for cup positioning and subsequent titration of femoral shortening in the reconstruction of Crowe IV dysplastic hips with degenerative joint disease. [Orthopedics. 2020; 43(2):e119–e122.]

When performing total hip arthroplasty (THA) for patients with end-stage degenerative joint disease associated with developmental dysplasia of the hip (DDH), cup placement may pose a challenge. Potential cup positions include the false acetabulum (pseudoacetabulum) or the true acetabulum (native acetabulum) and depend on factors such as acetabular morphology and bony deficiency. Where possible, the true acetabulum should be sought, as this leads to superior biomechanics, improved fixation, and hip stability.1,2 However, localization of the true acetabulum may prove challenging because of soft tissue contractures, occlusion of the cotyloid fossa, and the high-riding femoral head.2,3

In turn, cup position affects the amount of femoral shortening necessary; together, both will affect final leg length. Traditionally, preoperative imaging has been used to plan the new hip center and the extent of shortening, to be confirmed by later intra- or postoperative imaging.

Traditional freehand or mechanical guide-assisted cup placement is an option. However, even with the use of mechanical guides, these techniques have limited accuracy.4,5

Imageless navigation is a novel tool that can be used to localize the true acetabulum intraoperatively, increasing the accuracy of cup placement.6

The authors describe a patient with Crowe IV DDH who underwent THA with cup placement at the true acetabulum as identified by preoperative computed tomography imaging and confirmed with intraoperative 3-dimensional mini-optical navigation (Intellijoint HIP; Intellijoint Surgical, Inc, Waterloo, Ontario, Canada). She then underwent femoral shortening based on the amount of distalization of the acetabular component.

Case Report

A 22-year-old woman with DDH presented with chronic mechanical left groin pain accompanied by painful grinding during position changes. Her symptoms were aggravated by walking long distances, and she noticed shortening of her extremity. Her parents recalled that she had undergone acetabular surgery at 3 years of age.

On examination, she was 163 cm tall, weighed 60 kg, and had a body mass index of 22.9 kg/m2. There was a curvilinear anterior hip scar. When standing, her left hemipelvis was minimally depressed and she was most comfortable with 1-cm and 1.5-cm blocks under her left heel. Left hip range of motion was 0° to 60° of flexion, internal rotation to −5° (with obligate external rotation on flexion), and external rotation to 15°. Straight leg raise elicited groin pain, and the Stinchfield test yielded positive results. Right hip range of motion was 0° to 120° of flexion, internal rotation to 10°, and external rotation to 40°. She walked with a coxalgic gait and tolerated a 1-cm block under the ipsilateral limb.

Radiographs and computed tomography scans revealed Crowe IV DDH with end-stage arthritic changes (Figure 1). The false acetabulum was located 34 mm superior and 26 mm lateral to the true acetabulum. The true acetabulum showed reduced medial wall thickness and anterior and superolateral deficiency. Ipsilateral tibial hypertrophy was also noted, minimizing leg length discrepancy.

Preoperative anteroposterior radiograph (A), 3-dimensional computed tomography reconstruction (B), supine computed tomography scanogram (C), and standing EOS (EOS Imaging, Inc, Cambridge, Massachusetts) radiograph (D).

Figure 1:

Preoperative anteroposterior radiograph (A), 3-dimensional computed tomography reconstruction (B), supine computed tomography scanogram (C), and standing EOS (EOS Imaging, Inc, Cambridge, Massachusetts) radiograph (D).

Intraoperatively, the camera platform was first anchored to the iliac crest (Figure 2).7 A tracking disc was placed on the posterolateral greater trochanter to facilitate coupling of the tracking array. The anterior pelvic plane and old hip center were then registered. Following neck resection, the cup was placed in the previously templated position with the aid of navigation. The resected femoral head was repurposed as autograft to fill the superolateral defect (Figure 2). The final position of the new hip center was 34 mm inferior, 29 mm medial, and 17 mm anterior (Figure 2). Subtrochanteric shortening of 20 mm was then performed, to achieve final limb lengthening of 14 mm. The intercalary segment was bivalved and onlaid onto the osteotomy site as graft (Figure 2). Postoperative radiographs verified cup placement according to the preoperative plan (Figure 3).

Intraoperative 3-dimensional mini-optical navigation setup showing camera platform attached to the iliac crest and femur platform attached to the greater trochanter (A). Subtrochanteric osteotomy: pre-osteotomy (B) and post-osteotomy (C).

Figure 2:

Intraoperative 3-dimensional mini-optical navigation setup showing camera platform attached to the iliac crest and femur platform attached to the greater trochanter (A). Subtrochanteric osteotomy: pre-osteotomy (B) and post-osteotomy (C).

Three-month postoperative anteroposterior (A) and cross-table lateral (B) radiographs and anteroposterior (C) and lateral (D) standing EOS (EOS Imaging, Inc, Cambridge, Massachusetts) radiographs.

Figure 3:

Three-month postoperative anteroposterior (A) and cross-table lateral (B) radiographs and anteroposterior (C) and lateral (D) standing EOS (EOS Imaging, Inc, Cambridge, Massachusetts) radiographs.

The patient was neurologically intact postoperatively and was pleased with her surgical outcome. She started 30% partial weight-bearing physical therapy on the first postoperative day and was discharged 3 days later. At 3 months, she still had mild groin and thigh pain with initial weight bearing that subsided with walking, as well as early fatigability. She had straight leg raise to 30°, hip flexion to 90°, abduction to 45°, and external and internal rotation to 30° and 10°, respectively. Radiographs and full-length EOS imaging (EOS Imaging, Inc, Cambridge, Massachusetts) showed good implant, bone graft, and cable position and a level pelvis (Figure 3).

Discussion

When performing THA in the setting of DDH with a high dislocation, final leg length is determined by cup position and femoral shortening.8 Placement in the true acetabulum is biomechanically superior and minimizes loosening and revision rates for both acetabular and femoral components.1 In contrast, chasing the pseudo-acetabulum results in a cup placed high, lateral, and oversized, leading to problems with fixation, stability, and restoration of hip biomechanics.2

It may be difficult to identify the true acetabulum because of interposed soft tissue and surrounding soft tissue contracture. Even with preoperative planning and intraoperative fluoroscopy, when inserted freehand, up to 25% of acetabular implants may end up outside the true acetabulum.2,3,9

Having found the true acetabulum, acetabular deepening, including controlled medial wall perforation (cotyloplasty), may be desirable to improve fixation, reduce superolateral cup uncoverage, and improve joint reaction forces. With a freehand approach, it may be difficult to control medialization because of deficient bony landmarks and poor medial bone stock.

Various enabling technologies have been used in the setting of DDH. Patient-specific templates and computed tomography–based navigation have been shown to improve acquisition of cup alignment and spatial positioning targets.10,11

Imageless navigation can improve the precision of cup distalization and medial–lateral offset. With the camera (iliac crest) and tracking array (posterolateral greater trochanter) as fixed points, the software identifies the old hip center prior to dislocation and neck resection and the new hip center after implant placement, with the difference equal to the change in spatial position.

This technology has been validated against computed tomography imaging and has been shown to be accurate to a mean of 0.74° and 0.97° for cup anteversion and inclination, respectively.12 Other studies found leg length and offset to be within 0.27 and 1.75 mm, respectively, compared with postoperative radiographs.12,13 Other authors have found this technology to be useful when performing THA for Legg–Calve–Perthes disease, another condition commonly associated with leg length discrepancy.14

Imageless optical navigation is less cumbersome than traditional navigation and is governed by a laptop computer placed outside of the surgical field. Therefore, it occupies a smaller footprint compared with robot-based systems, where surgical assistants may have to step aside for the robot arm. It obviates the need for preoperative axial imaging and registration of bony anatomy with a probe. Further, unlike traditional navigation, it involves no capital purchase and minimal setup time.6,13 The freestanding tracking array can also be coupled with a probe to aid in confirmation of cup inclination and anteversion. This functionality may be desirable in the revision setting. It may supplement axial imaging in evaluating existing cup position in patients with chronic dislocation. Potential risks and disadvantages of the technology include registration errors, pin-site complications, and tracker dislodgement.

Femoral shortening is commonly planned preoperatively, with final shortening determined intraoperatively, depending on final cup position. Krych et al15 recommend bone resection corresponding to femoral bony overlap following the initial transverse osteotomy. This method is less precise and dependent on soft tissue tension and the enthusiasm of the surgical assistant. With navigation, the surgeon is able to determine the exact amount of cup distalization and thus the concomitant amount of femoral shortening necessary to attain the desired leg length.

Conclusion

This case illustrates the utility of imageless optical navigation in cases of DDH undergoing THA. Navigated cup placement facilitates acquisition of pre-operative spatial targets and, in turn, titration of the amount of femoral shortening necessary for the intended amount of limb lengthening.

References

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Authors

The authors are from Adult Reconstruction and Joint Replacement (CYW, EPS), Hospital for Special Surgery, New York, New York; and Intellijoint Surgical, Inc (JMM), Waterloo, Ontario, Canada.

Dr Woon has no relevant financial relationships to disclose. Dr Muir is a paid employee of Intellijoint Surgical, Inc. Dr Su has received grants from Smith & Nephew and United Orthopedics, Inc; is a paid consultant for and receives research support from Smith & Nephew and United Orthopedics, Inc; and holds stock in Orthalign, Inc.

Correspondence should be addressed to: Colin Yi-Loong Woon, MD, Adult Reconstruction and Joint Replacement, Hospital for Special Surgery, 535 E 70th St, New York, NY 10021 ( colinylwoon@gmail.com).

Received: April 16, 2019
Accepted: September 09, 2019
Posted Online: December 31, 2019

10.3928/01477447-20191223-06

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