Robotic-Assisted TKA in a Patient with Multiple Hereditary Exostoses
A 57-year-old male with multiple hereditary exostoses (MHE) presents with worsening left knee pain and a valgus deformity due to significant osteoarthritis. What is the best way to perform a total knee arthroplasty, given his MHE and extra-articular deformities of the femur and tibia?
Samantha L. Harrer, BA; John R. Schnell, MD; and P. Maxwell Courtney, MD
Multiple hereditary exostoses (MHE) is an autosomal dominant disease characterized by osteochondromas, or exostoses, caused by a point mutation in the exostosin family of genes.  Osteochondromas are cartilage-capped bony outgrowths that have the potential to cause joint deformities, restricted range of motion (ROM), and early-onset osteoarthritis (OA), depending on the location. 
The knee is the most affected joint in MHE, and up to one third of patients develop genu valgum.  Various surgical management options have been described in the literature, with a focus on correction of valgus deformity. Interventions include exostoses resection, joint deformity rectification, and limb-length discrepancy alignment.  Total knee arthroplasty (TKA) has rarely been utilized for deformity correction in patients with MHE and coexisting severe OA. [4,5]
The literature on TKA in patients with MHE is limited, and few cases of TKA in combined extra-articular deformity and OA have been described. Of these cases, varied methods and approaches have been utilized. [6,7] For example, Grzelecki et al  describe a 1-stage TKA and tibial shaft osteotomy with good outcomes. 
Using conventional TKA instrumentation in these patients may be quite challenging, given the deformity and the inability to instrument the canals. Several techniques have been described for TKA in patients with extra-articular deformities, including patient-specific instrumentation, navigation, and extramedullary cutting guides. Recently, robotic-assisted TKA has been used to manage these difficult cases.
In this case report, we describe the operative management of a patient with MHE and severe OA using a CT-based, robotic-assisted TKA procedure. With the assistance of technology, we hoped to avoid performing an osteotomy or substantial soft-tissue releases that would necessitate the use of a constrained implant.
A 57-year-old male with a past medical history of MHE presented with worsening left knee pain of more than 3 years’ duration. He had failed extensive conservative treatment that had included corticosteroid injections, anti-inflammatory medication, and physical therapy.
The patient has had numerous open knee surgeries on his left knee to remove osteochondromas, including a resection of his proximal fibula. He has had a progressive genu valgum deformity and instability. Past surgical history also includes a right total hip arthroplasty and a left hip osteotomy 20 and 25 years prior, respectively.
Additional medical history is significant only for hypertension.
- Height: 5 feet, 2 inches; weight: 190 pounds; body mass index: 34.74 kg/m2
- Antalgic gait
- Severe lateral joint line tenderness, effusion, and crepitus present
- Knee stable with range of motion (ROM) from 5° to 105°
- Valgus alignment partially correctable but does not overcorrect
- Normal left extremity motor and sensory exam, with palpable pedal pulses
Figure 1. Preoperative standing anteroposterior left knee (top left), standing flexed left knee (top right), and sunrise view (bottom) radiographs demonstrate severe OA that is worse in the lateral compartment. The patient also has multiple osteochondromas and extra-articular deformities on the left distal femur and proximal tibia.
Figure 2. Full-length standing radiographs demonstrate mechanical limb alignment of 11° of valgus from the anatomic axis. The patient has a varus extra-articular deformity of the distal femur and a valgus deformity of the proximal tibia.
- Left knee OA
- MHE with extra-articular deformities of the distal femur and proximal tibia
Because the patient had exhausted conservative management options for his left knee, he wished to proceed with TKA. Given his history of osteochondroma excisions and his extra-articular deformities, we discussed corrective osteotomy prior to TKA. This would involve osteotomies of his distal femur and proximal tibia, as well as a prolonged period of non-weight-bearing, and would be a significant undertaking due to his poor bone quality for fixation.
We discussed the possibility of performing a TKA only, telling the patient that we believed the procedure would be able to correct his alignment, given that the full-length standing radiographs showed his overall mechanical alignment in only 11° of valgus. However, we would not be able to use conventional TKA instrumentation because of his extra-articular deformities and prior hip osteotomy, our inability to instrument the femoral canal, and the compensatory valgus deformities of both his ankles. Extra-medullary guides with intraoperative fluoroscopy could be used but would not be as precise. We also discussed the need for constraint and stemmed implants.
We recommended robotic-assisted TKA (Mako, Stryker; Kalamazoo, Michigan), which would help with precise component positioning, correct his mechanical axis, and minimize the need for major soft-tissue releases and constrained, stemmed components. The patient agreed to this procedure.
- Prior to surgery, we obtained a CT scan of the patient’s hip, knee, and ankle per the robotic system’s protocol. The scan was then used to develop an individualized 3D model of the knee.
- Although conventional alignment has long been the standard, we chose to utilize individualized alignment for this case. Individualized alignment follows principles of kinematic alignment, using robotic-assisted technology for anatomic-oriented implant placement and only minor changes to soft-tissue balancing. This potentially decreases the need for more-aggressive soft-tissue releases.
- The initial preoperative plan was to restore the patient’s distal femoral alignment, starting with resection of 5 mm from the medial and lateral sides. The patient had some medial distal femoral bone loss, so the minimal resection would take into account the missing bone and cartilage to avoid raising raise the joint line
- The plan included restoration of the physiologic valgus of the distal femur, with native femoral rotation relative to the posterior condylar axis (in this case, 0.5° internal) or the transepicondylar axis (in this case, 1.0° internal).
- The femoral component was appropriately sized to match the sagittal profile of the femur and flexed to avoid notching. We could tolerate a small anterior femoral notch in this patient due to his MHE.
- The tibia resection was set with the overall mechanical axis, displayed at the upper left corner of Figure 3; in our case, 1.0° of overall varus alignment.
Figure 3. Preoperative planning for TKA utilizing individualized alignment.
- With the preoperative plan set, we took the patient to the operating room.
- We performed a standard medial parapatellar approach, utilizing the patient’s old anterior incision. Unicortical tracking pins were placed in the femur and tibia.
- The bony surface was registered using a sharp probe.
- After the anterior cruciate ligament and osteophytes were removed, we assessed the soft tissue balance in 90° of flexion and 90° of extension (Figure 4). We chose to preserve the posterior cruciate ligament. The knee was stressed to both varus and valgus stress.
Figure 4. Intraoperative assessment of soft tissue balancing.
- Small changes to the preoperative plan were then made to achieve a symmetric flexion and extension gap of 18 mm to 19 mm, which accommodated the 8-mm thickness of the distal femoral component and the 9-mm minimal polyethylene thickness.
- The robotic arm was brought in to make the cuts. The trials were then placed, and the knee was found to be very well balanced (Figure 5). Major soft-tissue releases and constrained implants were not needed.
Figure 5. Postoperative knee state with symmetric 18–mm gaps in extension and 19–mm gaps in flexion. The overall knee alignment was within 2° of a neutral mechanical axis.
- We used a cementless implant due to the patient’s young age and surprisingly good bone quality at the joint line.
- The patella was cut to a size 29-mm asymmetric patella button using a free-hand technique.
- The components were impacted into place with a 9-mm cruciate-retaining polyethylene insert.
- Postoperative radiographs are seen in Figure 6.
Figure 6. Postoperative radiographs of the left knee demonstrating a cementless left TKA.
Postoperative Follow Up
- The patient was allowed to weight-bear as tolerated and was discharged home on POD1. His discharge orders included 81 mg of aspirin twice daily for prophylaxis of deep vein thrombosis.
- At 6 weeks, the patient had a well-healed incision with excellent stability and ROM from 0° to 110°.
- Patients with MHE present unique treatment challenges when considering TKA, including extra-articular deformities and osteochondromas.
- Full-length alignment films should be obtained on all patients to assess their mechanical alignment and any deformities.
- Use of traditional instrumentation can be quite challenging in these cases. Robotic-assisted TKA can help the surgeon make more precise cuts, anatomically position the components, and more reliably restore overall alignment.
- Individualized alignment follows kinematic principles and may help reduce the need for soft-tissue releases and constraint, even in these difficult cases. Although early results are promising, further study is needed to assess long-term outcomes. 
- With individualized alignment techniques, the proximal tibia is often cut in varus with internal rotation of the femoral component due to a physiologic valgus distal femur. We recommend limiting varus on the tibia and femoral internal rotation related to concerns about patellar maltracking and implant failure. Our preference is a maximum of 3°; however, further study is needed to define these boundaries.
Samantha L. Harrer, BA, is a medical student at Thomas Jefferson University, Sidney Kimmel Medical College in Philadelphia, Pennsylvania. John R. Schnell, MD, is an orthopaedic surgeon at The Rothman Orthopaedic Institute, Philadelphia, Pennsylvania. P. Maxwell Courtney, MD, is an orthopaedic surgeon at The Rothman Orthopaedic Institute and an Associate Professor of Orthopaedic Surgery at Thomas Jefferson University, Philadelphia, Pennsylvania. Dr. Courtney is also the Adult Reconstruction Section Editor of Rothman Institute Grand Rounds on ICJR.net.
Disclosures: Ms. Harrer and Dr. Schnell have no disclosures relevant to this article. Dr. Courtney has disclosed that he is a paid consultant for Stryker and Zimmer Biomet, who utilize robotic-assisted technology for TKA.
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