A Novel Vector-Based Computer Tomography Alignment Measurement Protocol for Total Knee Arthroplasty.

INTRODUCTION: Component position and overall limb alignment following total knee arthroplasty (TKA) have been shown to influence implant survivorships and clinical outcomes. While most surgeons utilize standard x-ray imaging for preoperative joint assessments, computer tomography scans (CT), coupled with automated digital analyses have been shown to provide additional surgical and clinical benefits. However, to date, a postoperative CT measurement protocol has not been reported for robotic-arm assisted TKA (RATKA). Therefore, the purpose of this paper was to assess the validity of a novel, vector-based CT alignment measurement protocol. Specifically, we compared: 1) final versus planned component alignment and placement; 2) inter-observer reliability; and 3) intra-observer reliability. MATERIALS AND METHODS: The CT-based technique utilized mathematical models to calculate prosthetic alignments from anatomical landmarks. To assess the models, 30 CT scans from multiple centers were collected on RATKA patients at six weeks postoperatively and analyzed using the proposed technique. Results were compared to the surgeons' preoperative plans for accuracy. Analyses were performed on the same protocol to determine inter-observer reliability. These analyses were repeated 30 days later to assess for intra-observer variability. RESULTS: The mean measurement errors compared between final versus planned component positions and alignments were: 0.79±1.48o varus in overall limb alignment (p=0.004); 0.34±1.20o varus (p=0.121); and 0.35±1.15o varus (p=0.17) for femoral and tibial varus/valgus alignment; 0.71±1.77o flexion (p=0.18) and 0.38±1.88o posterior (p=0.41) for femoral flexion and tibial slope. There was strong reproducibility between observers. Correlation analyses showed low variabilities, with slopes between 0.8 to 1.0 and all R>0.8. CONCLUSION: As robotic technologies become widely available in orthopaedic surgery, it is critical to have tools, such as CT protocols, which can quantitatively verify operative decisions concerning limb alignment and component placement. This study described a novel, vector-based, CT alignment measurement protocol for RATKA which has not previously been defined. The method demonstrated excellent accuracy to plan and low intra- and inter-observer variability. This is a valuable analysis tool for RATKA studies where component accuracy is assessed using postoperative CT images.

Robotic-Arm Assisted Total Knee Arthroplasty Demonstrated Soft Tissue Protection.

INTRODUCTION: While total knee arthroplasty (TKA) procedures have demonstrated clinical success, occasionally intraoperative complications can occur. Collateral or posterior cruciate ligament injury, instability, extensor mechanism disruption, and tibiofemoral or patellofemoral dislocation are among a few of the intraoperatively driven adverse events prevalently ranked by The Knee Society. Robotic-arm assisted TKA (RATKA) provides a surgeon the ability to three-dimensionally plan a TKA and use intraoperative visual, auditory, and tactile feedback to ensure that only the desired bone cuts are made. The potential benefits of soft tissue protection in these surgeries need to be further evaluated. The purpose of this cadaver study was to assess the a) integrity of various knee soft tissue structures (medial collateral ligament [MCL], lateral collateral ligament [LCL], posterior cruciate ligament [PCL], and the patellar ligament), as well as b) the need for tibial subluxation and patellar eversion during RATKA procedures. MATERIALS AND METHODS: Six cadaver knees were prepared using RATKA by a surgeon with no prior clinical robotic experience. These were compared to seven manually performed cases as a control. The mean Kellgren-Lawrence score was 2.8 (range, 0 to 4) in RATKA and 2.6 (range, 1 to 4) in the manual cohort. The presence of soft tissue damage was assessed by having an experienced surgeon perform a visual evaluation and palpation of the PCL, MCL, LCL, and the patellar ligament after the procedures. In addition, leg pose and retraction were documented during all bone resections. The amount of tibial subluxation and patellar eversion was recorded for each case. RESULTS: For all RATKA-assisted cases, there was no visible evidence of disruption of any of the ligaments. All RATKA cases were left with a bone island on the tibial plateau, which protected the PCL. Tibial subluxation and patella eversion were not required for visualization in any RATKA cases. In two of the seven MTKA cases, there was slight disruption noted of the PCL, although this did not lead to any apparent change in the functional integrity of the ligament. All MTKA cases required tibial subluxation and patellar revision to achieve optimal visualization. DISCUSSION: Several aspects of soft tissue protection were noted during the study. During bone resections, the tibia in RATKA procedures did not require subluxation, which may reduce ligament stretching or decrease complication rates. Potential patient benefits for short-term recovery and decreased morbidity to reduce operative complications should be studied in a clinical setting. Since RATKA uses a stereotactic boundary to constrain the sawblade, which is generated based on the implant size, shape, and plan, and does not have the ability to track the patient's soft tissue structures, standard retraction techniques during cutting are recommended. Therefore, the retractor placement and potential for soft tissue protection needs to be further investigated. RATKA has the potential to increase soft tissue protection when compared to manual TKA.

In vivo kinematics of a robot-assisted uni- and multi-compartmental knee arthroplasty.

BACKGROUND: There is great interest in providing reliable and durable treatments for one- and two-compartment arthritic degeneration of the cruciate-ligament intact knee. One approach is to resurface only the diseased compartments with discrete unicompartmental components, retaining the undamaged compartment(s). However, placing multiple small implants into the knee presents a greater surgical challenge than total knee arthroplasty, so it is not certain that the natural knee mechanics can be maintained or restored. The goal of this study was to determine whether near-normal knee kinematics can be obtained with a robot-assisted multi-compartmental knee arthroplasty. METHODS: Thirteen patients with 15 multi-compartmental knee arthroplasties using haptic robotic-assisted bone preparation were involved in this study. Nine subjects received a medial unicompartmental knee arthroplasty (UKA), three subjects received a medial UKA and patellofemoral (PF) arthroplasty, and three subjects received medial and lateral bi-unicondylar arthroplasty. Knee motions were recorded using video-fluoroscopy an average of 13 months (6-29 months) after surgery during stair and kneeling activities. The three-dimensional position and orientation of the implant components were determined using model-image registration techniques. RESULTS: Knee kinematics during maximum flexion kneeling showed femoral external rotation and posterior lateral condylar translation. All knees showed femoral external rotation and posterior condylar translation with flexion during the step activity. Knees with medial UKA and PF arthroplasty showed the most femoral external rotation and posterior translation, and knees with bicondylar UKA showed the least. CONCLUSIONS: Knees with accurately placed uni- or bi-compartmental arthroplasty exhibited stable knee kinematics consistent with intact and functioning cruciate ligaments. The patterns of tibiofemoral motion were more similar to natural knees than commonly has been observed in knees with total knee arthroplasty. Larger series are required to confirm these as general observations, but the present results demonstrate the potential to restore or maintain closer-to-normal knee kinematics by retaining intact structures and compartments.

Kinematic and contact stress analysis of posterior malleolus fractures of the ankle.

OBJECTIVE: To determine if there are measurable dynamic contact stress aberrations and kinematic abnormalities (instability) that have not been observed in conventional static loading studies of posterior malleolar ankle fractures. DESIGN: Cadaveric fracture model. SETTING: Biomechanics laboratory. INTERVENTION: Seven fresh cadaveric specimens were fixed in an unconstrained testing apparatus and loaded to one body weight. The ankle was moved from 25 degrees of plantarflexion to 15 degrees of dorsiflexion. The model included the intact ankle and four fracture simulations (50% fracture without internal fixation, 2 mm gap and step malreductions, and anatomically fixed). MAIN OUTCOME MEASURE: Motion at the ankle was monitored with an electromagnetic tracking device, and intra-articular contact stresses were measured using a real-time stress sensor. RESULTS: There were no kinematic abnormalities suggestive of tibiotalar subluxation in any of the fracture simulations. There was no increase in peak contact stress in any of the fracture models compared with the unfractured model. However, there was a shift in the location of the contact stresses to a more anterior and medial location following the fracture. When summed over the range of motion, these areas of cartilage bore significantly higher cumulative contact stresses relative to the nonfracture situation. CONCLUSIONS: We found no talar subluxation and no increase in contact stresses near the articular incongruity, making it unlikely that these factors explain the increased incidence of arthrosis after trimalleolar fractures (OTA/AO classification 44 B3 fractures). Rather, we found that the joint remaining bears increased stress and that the center of stress shifts anteriorly, loading cartilage that normally sees little load.

Gait cycle finite element comparison of rotating-platform total knee designs.

Functional load transmission and kinematic performance were compared for standard versus posterior-stabilized versions of a rotating-platform total knee implant, over a standardized loading cycle, using three-dimensional contact finite element analysis. These two design variants differ primarily in terms of the latter's polyethylene insert having a cam that engages with the femoral component during appreciable flexion, thereby inducing femoral component rollback. The finite element model, previously validated experimentally, afforded direct comparisons of anterior lift-off of the insert from the tibial tray, of bearing mobility (insert rotation about the pivot post), of femoral rollback, and of metal-on-polyethylene contact stresses at the bearing and backside surfaces of the insert. Both design variants generally performed comparably, exhibiting an internal and external rotation range of approximately 5 degrees, approximately 1.5 mm peak lift-off at the anterior aspect of the insert, and approximately 15 mm of posterior rollback, the respective maxima for both designs occurring at approximately the same instants in the gait cycle. However, the posterior-stabilized design had slightly more rollback, and slightly less anterior lift-off and rotation, than did the standard rotating-platform design. Peak polyethylene stresses occurred on the backside of the insert near the posterior edge of the medial compartment, the magnitude being approximately 18% higher for the posterior-stabilized design (21 MPa) than for the standard design.