1 Introduction

The incidence of hip fractures is growing alongside the aging population, with nearly 580,000 hip fractures estimated to occur in the US every year by 2040.¹ Multiple surgical treatment options are available for hip fracture fixation, including cannulated screws, femoral neck system, intertrochanteric nails, sliding hip screws, hip hemiarthroplasty, and total hip arthroplasty (THA), but postsurgical complications do occur. The complications include nonunion, malunion, periprosthetic fracture, and post traumatic osteoarthritis.^2,3 While these treatments have high rates of restoring hip stability in the acute setting, posttraumatic hip arthritis can lead to decreased mobility and increased pain. For these patients, conversion total hip arthroplasty (cTHA), which consists of the removal of the implant utilized for hip fracture fixation, is indicated.

Compared to primary THA, cTHA has greater complexity and increased complication rates. In cTHA, all or most of the original implant needs to be removed before the new implant can be implanted. Often, hardware and structural abnormalities that develop over time must also be properly addressed.³ Removing the original implant and addressing additional complications prolongs the surgical time for cTHA.² Surgical site infections are twice as likely to occur in cTHAs than in primary THAs.³ A retrospective study performed in 2018 revealed that 7.7 % of converted THAs resulted in periprosthetic joint infections, while only 1.4 % of primary THAs resulted in periprosthetic joint infections.⁴ The longer surgical time associated with cTHA and the addition of new hardware may contribute to the greater incidence of surgical site infections in these patients than in primary THA patients.^3,5 Previous studies have established that longer surgical times and the installation of new hardware in previously operated areas add to the already increased risk of infection, and the combination of both may contribute to a greater rate of infection.²

Blood loss is greater in patients who undergo cTHA than in those who undergo primary THA, which can be attributed to the formation of scar tissue following the original operation.^6,7 Compared with primary THAs, cTHAs require double the amount of blood transfusions.⁴ Most notably, postoperative dislocation and fracture rates are greater in cTHAs than in primary THAs.¹

Conversion THAs pose more surgical and postoperative challenges than primary THAs, but recent surgical innovations show potential for mitigating these risks. Robotic-assisted THA may prove to be a viable option for helping to alleviate the challenges and complications associated with cTHA through intraoperative navigation, implant position and gap data, and robotic arms assisted with haptic boundaries.⁸ Concerns exists regarding the distortion of the preoperative CT imaging required for use of robotic assistance, which is due to the existing metal implants placed for hip fracture fixation. Currently, the Mako system is FDA approved for primary total hip arthroplasty. Utilization of the system for cTHA is an off-label use.

We present a series of two cases in which the Mako robotic arm system was used for cTHA after the removal of intramedullary and extramedullary implants involving the proximal femur. The technique described in these two cases represents off-label use of the Mako total hip arthroplasty system. Informed consent from both patients was obtained prior to this case series.

2 Case series

2.1

2.1 Case 1

A 64-year-old male with a past medical history of left intertrochanteric fracture sustained after a motor vehicle accident 40 years prior presented for evaluation of progressive left hip pain for many years with 0–80 degrees of flexion and no internal/external rotation. Nonoperative management, including nonsteroidal anti-inflammatories, physical therapy and corticosteroid injection, was tested but was found to be ineffective for symptom relief. X-ray imaging of the left hip obtained at the initial appointment (Fig. 1-A and 1-B) revealed severe osteoarthritis in the setting of a healed left intertrochanteric fracture with a sliding hip screw. The patient elected to undergo combined hardware removal and total hip arthroplasty of the left hip. Robot assistance (MAKO, Stryker Medical) was utilized for the procedure.

Close

Fig. 1 A (AP view) and 1-B (lateral view) of the left hip at the initial visit.

Export to PPT

The removal of hardware and THA were performed sequentially as a single procedure. The patient was placed in the right lateral decubitus position. A lateral Hardinge approach was utilized with normal array placement. Next, the three cortical shaft screws were removed from the sliding hip screw plate. Once the shaft screws were removed, removal of the sliding hip screw was attempted. During removal, a fracture propagated from the lateral femur cortex at the superior aspect of the plate, extending to the medial cortex just distal to the lesser trochanter. Femoral neck osteotomy was then performed in preparation for the insertion of the femoral arthroplasty component and for removing the sliding hip screw and plate together in a retrograde fashion. After the registration, preparation and reaming of the acetabulum, an uncemented 54-mm acetabulum shell (Trident II Tritanium, Stryker) and a size E 42-mm polyethylene liner were inserted at 16 degrees of anteversion and 40 degrees of abduction. A single luque wire was used for temporary fixation of the intraoperative femur fracture. Using robot assistance, a second osteotomy was performed to prepare the femur for implantation. The femur was prepared for the insertion of an uncemented 19-mm x 155-mm diaphyseal fitting stem (Restoration Modular Stem, Stryker). The MDM bipolar head was then impacted onto the femur component. Three cables, 2 cables around the medial fracture spike and 1 cable placed superior to the lesser trochanter, were then placed for permanent stabilization of the intraoperative fracture. Lateral cortical bone defects from the removed sliding hip screw were filled with autologous bone that was lost during the initial removal of the sliding hip screw. X-ray imaging of the left hip was obtained, confirming appropriate placement of the components (Fig. 2-A, 2-B). The patient was discharged on postoperative day 2 in good condition. At the 1-year follow-up, the patient reported significant improvements in pain in the left hip and overall quality of life.

Close

Fig. 2 A (AP view) and 2-B (lateral view) of the left hip, postoperative day 0.

Export to PPT

Written informed consent was obtained prior to case submission and publication.

2.2

2.2 Case 2

A 49-year-old female was evaluated for left hip pain for 10 years. Symptom onset occurred after intramedullary nailing of the proximal left femur, which was indicated for treatment of a cystic lesion at the left femoral neck that was diagnosed as fibrous dysplasia. Other conservative therapies, which included physical therapy, joint injection, and over-the-counter analgesics, provided minimal to no relief of her symptoms. The patient's range of motion at the left hip was 100 degrees of flexion, 0 degrees of extension, 20 degrees of internal rotation, and 20 degrees of external rotation. X-ray images of the left hip at the initial visit were obtained (Fig. 3-A, 3-B), which revealed severe osteoarthritis in the setting of the intramedullary nail with no evidence of hardware failure. After discussion between the orthopedic surgeon and patient, the patient elected to undergo combined removal of hardware and total hip arthroplasty of the left hip with robot assistance (MAKO, Stryker Medical).

Close

Fig. 3 A (AP view) and 3-B (lateral view) of the left hip at the initial visit.

Export to PPT

The removal of hardware and THA were performed sequentially as a single procedure. The patient was placed in the right lateral decubitus position. A lateral Hardinge approach was utilized to expose the hip joint. Schanz pins were then placed in the anterior iliac crest for robotic array sensor placement. Removal of the set screw from the intramedullary nail was addressed first, requiring removal of the overlying bone near the greater trochanter. Once removed, the location of the greater trochanter was established by the Mako robot. After registration, leg length and offset were assessed by the surgeon and Mako software. Next, the femoral head was dislocated from the acetabulum. The lag screw in the femoral head was removed with some difficulty, requiring use of trephines along the lateral cortex and splitting the femoral head and neck longitudinally in multiple sections. This is important to note and anticipate the placement of the femoral checkpoint. Next, careful rongeuring of the bone near the greater trochanter was needed. Great care was taken to ensure the integrity of the greater trochanter during extraction of the intramedullary nail. Once all screws were removed, the intramedullary nail was removed from the femoral canal. All components of the intramedullary nail were successfully removed, and preparation of the hip joint for arthroplasty components was performed. The acetabulum was prepared and reamed with the guidance of the Mako robot. A size 52 reamer was utilized, with the inclination set at 40° and the anteversion at 16°. A 52-mm metal acetabulum (Trident II Tritanium Solid Back, Stryker) was impacted into the prepared acetabulum, followed by the insertion of a 36-mm high-density polyethylene liner. Once the acetabulum was complete, a box osteotome was used to open the femoral canal slightly medial to the path of the previously inserted intramedullary nail. The femoral canal was reamed to fit a 14-mm by 155-mm stem (Restoration Module Revision Hip System, Stryker). Because of the osseous changes from the extricated intramedullary nail, a standard offset proximal femur cone was placed to increase the overall stability of the construct. The final construct was tested for stability and range of motion, and X-ray imaging (Fig. 4-A, 4-B) was performed to evaluate the appropriate positioning of the implants prior to closure. The immediate postoperative hospital stay was complicated by acute blood loss anemia, requiring transfusion of two units of packed red blood cells on postoperative day one. No other complications occurred during the two years of postoperative follow-up. The patient reported significant improvement in pain and quality of life after the procedure.

Close

Fig. 4 A (AP view) and 4-B (lateral view) of the left hip, postoperative day 0.

Export to PPT

3 Discussion

Conversion THA is a complex procedure often accompanied by many short- and long-term complications.⁹ Because the use of the MAKO robotic arm system for cTHA is limited, we believe this is the first report of the benefits of the MAKO system in cTHA.

Successful cTHA requires accurate preoperative planning, which is dependent on the quality of the preoperative CT imaging of the operative joint. The data obtained can help to identify important anatomical landmarks, hardware positioning, and bone malformations.¹⁰ Proper imaging and data collection allows for the optimal component size and orientation of the new implant while ensuring successful removal of the previous hardware.¹¹

In the first presented case, the MAKO robotic system was used to perform an osteotomy in the femur in preparation for the implementation of the MDM bipolar head. Normally, osteotomy is performed by a surgeon utilizing an oscillating saw guided by preoperative X-ray imaging and CT scans.¹² The utilization of the robot to perform the osteotomy in this procedure proved to be successful, as the intraoperative X-ray imaging showed accurate placement of all the components. Compared with conventional osteotomies, robot-operated osteotomies have been shown to reduce the osteotomy angular error by 23 % and distance error by more than 48 % in animal models.¹³

The use of the MAKO robot in the second patient was helpful in more accurate assessment of leg length and offset, as well as reaming of the acetabulum and its placement. Accurate assessment of leg length and proper acetabulum component placement are critical for successful primary and conversion THA.¹¹ Limb length discrepancy (LLD) is the most common cause of patient dissatisfaction and complications following primary and cTHA.¹⁴ Patients with LLDs after THA often report pain, gait deformities, and overall dissatisfaction. LLDs following THA can be caused by many variables, but the common sources are surgeon error, implant instability and incorrectly sized hardware.¹⁵ Several of the factors leading to LLDs can be addressed using robotic assistance. The use of robots in primary THAs has been shown to decrease postoperative LLDs.^15,16 The increased accuracy provided by the robot assistance measurements and placement contributed to the decreased incidence of LLDs. The use of the MAKO system has been shown to make more precise and accurate measurements than intraoperative manual measurements made by humans.¹⁷ Compared to manual acetabular component placement, robotic-assisted acetabular component placement is more likely to be within the Lewinnek safe zone by more than 30 %.¹⁸

As the use of the MAKO system is off label for use in cTHAs, there is little to no evidence of improved hardware placement and decreased LLDs in this category of operations. However, the present case showed the viability of the robot's use for this procedure and its successful outcomes, with patients reporting significant improvements in pain and quality of life.

The use of robots in primary THAs is well documented, with demonstrated technical improvements equating to improved patient outcomes. The performance of cTHAs with robotic assistance is not well documented, and little information is available, most likely due to its off-label usage. The two robotic assisted cTHAs presented had good to excellent outcomes in terms of pain and function, and post-operative imaging shows satisfactory radiographic parametric. The use of robotic assisted cTHAs, therefore, shows promise to improve patient outcomes. Various off-label robotic-assisted surgeries are gaining traction. For example, the use of the da Vinci Xi surgical robot as an off-label robot in transoral robotic surgery is rapidly growing, with many calling for it to become officially licensed.¹⁹ Additionally, the MAKO robotic system is increasingly being used in off-label procedures for total knee arthroplasty revisions and conversions.^20,21 Currently, the MAKO system is being utilized by surgeons for off-label use in THA revision operations.^22,23

4 Conclusion

Conversion total hip arthroplasty is becoming a more common procedure with increasing population age. In this case series, we presented two cTHAs utilizing the MAKO robotic system. Both operations were successful, with patients reporting improved quality of life and reduced pain. As innovation continues, we believe that robot assistance will become an invaluable tool utilized across all orthopaedic subspecialties.

CRediT authorship contribution statement

Michael Oravic: Data curation, Writing – review & editing. Oliver Sogard: Data curation, writing. Andrew D. Lachance: writing, Writing – review & editing. Michael Elder Waters: Data curation. Daniel Tayag: Writing – review & editing, creation of project. David Austin: Writing – review & editing, creation of project.

Consent

Human subjects research was deemed not applicable by IRB.

Ethics

There are no reported ethical conflicts of interest within this study.

Disclosure

No funds were received in support of this work. No benefits in any form have been or will be received from any commercial party related directly or indirectly to the subject of this manuscript.

Funding

There was no funding for this project.