Since its introduction, unicompartmental knee arthroplasty has been controversial because of poor early clinical outcomes due to implant design, bony fixation, surgical instrumentation, and technique. Improvements in surgical technique and implant design have resulted in improved results and greater survivorship. The ability to obtain accurate implant placement includes avoiding surgeon decisions leading to potential errors. These errors include alignment in the sagittal, coronal, and axial planes on each prepared condyle as well as the preservation of the joint line and the resulting overall limb alignment as something critical to obtaining a successful outcome.
Key points
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This new robotic procedure provides comprehensive, 3-dimensional planning of partial knee components, including soft tissue balancing, followed by accurate resection of the femur and the tibia. This preparation allows for precise placement and alignment of the components.
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Patients have shown significant improvements in their postoperative function in every functional measurement, including more normal knee kinematics.
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The introduction of new procedures and technologies in medicine is routinely fraught with issues associated with learning curves and unknown potential complications.
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Because the specific objectives of this novel technology are to optimize surgical procedures to provide more safe and reliable outcomes, the favorable results seen to date prove this technology to be a significant improvement in the surgical technique of partial knee arthroplasty.
Surgical Errors are fully in the surgeons’ control and have a prominent effect on implant survival. A surgeon must familiarize himself with the system of choice and perform a consistent amount of surgeries to achieve consistent outcomes. Unicompartmental Knee Replacement (UKR)s are very unforgiving to technical errors, which often lead to early postoperative failures.
Common Errors
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Overcorrection of alignment
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Flexion-extension instability
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Not achieving cortical rim coverage with the tibial tray/ or allowing overhang of components
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Patellar arthritis (grade 4) or instability
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Inset tibial poly tray may subside
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Poor cement technique
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Anterior placed femoral component
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Poor visualization can lead to
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Over/under resection of tibia
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Over/undercorrection of limb alignment
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Implant placement
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Tibia—under coverage of cortex, overhang
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Anterior Cruciate Ligament (ACL) injury
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Excessive slope
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Mal-rotation—internal rotation-external rotation (IR-ER) of implants
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Patellar impingement
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Cement retention
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Pin site fracture
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Retained osteophytes
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Introduction
In the late 1990s, Repicci and Eberle introduced a unicompartmental knee procedure using an inlay tibial component termed “minimally invasive surgery” ( Fig. 1 ). This procedure resulted in earlier mobilization, shorter inpatient stay, and shorter length of rehabilitation than had been observed for the conventional surgical approach. However, concerns were raised about loss of accuracy with minimally invasive techniques. With minimally invasive procedures, visualization is reduced leading to potential errors in implant placement, limb alignment, cement technique, and bone preparation ( Fig. 2 ).
The introduction of technology to improve accuracy of unicompartmental outcomes began with navigation. Jenny and colleagues reported on a series of 60 patients who underwent navigated minimally invasive unicompartmental knee arthroplasty (UKA) and 60 patients who underwent navigated larger incision UKA. These authors cited the advantages of reduced surgical trauma, reliability, and safety obtained with the navigated minimally invasive procedure, whereas radiographic accuracy of implantation was the same for both minimally invasive and larger incision navigation techniques.
Justin Cobb first introduced robotic assistance in 2000 using the Acrobot robot to improve the accuracy of implant positioning during UKA.
Cobb and colleagues first reported a prospective comparison of a tactile-guided robot-assisted UKA and conventional UKA performed with manual instrumentation. Their robotic system used static referencing that required rigid intraoperative fixation of the femur and tibia to a stereotactic frame. The primary outcome measurement was the angle of the tibiofemoral alignment in the coronal plane, measured by computed tomography (CT). Implant position errors relative to the planned position averaged 1.1 mm and 2.5° with robotic assistance compared with 2.2 mm and 5.5° conventionally along any axis. Overall tibiofemoral coronal plane alignment was within 2° for every case performed with robotic assistance. Only 40% of conventional surgery achieved this level of accuracy.
In 2006, MAKO Surgical Corporation obtained US Food and Drug Administration clearance to begin the first implantation of medial UKAs using a haptic-controlled passive robotic arm. This system allows an accurate surgical preparation of the specific patients’ diseased knee compartment in one or multiple compartments, sparing healthy tissue in the undiseased compartments. This allows the surgeon to tailor the surgical procedure to the patient’s arthritic and kinematic needs ( Fig. 3 ). Addressing the medial, lateral, and patellofemoral joint in an individual, or modular manner, can be planned preoperatively and refined intraoperatively as the patient’s knee kinematics and the disease pathology requires.
Modular implants allowed inlay tibial or onlay tibial components to be used, with an anatomic femoral and trochlear implant. The implant design was based on CT-based parameters of more than 100 healthy and diseased knees. The femoral and patellofemoral components are created from a single, continuous surface. The inner geometries are contoured to allow bony preparation using a 6-mm burr. The femoral component allows deep flexion and has angled pegs to improve implant stability. The patellar component used in bicompartmental knee arthroplasty is a dome-shaped all polyethylene 3-pegged design, not pictured ( Fig. 4 ).
The integration of robotic-arm assistance allows a surgeon to have a controlled, highly accurate system that is be integrated into the present surgical workflow. The enabling technology in this robotic platform was the development of haptics. Haptics is the science of applying touch (tactile) sensation and control to interaction with computer applications ( Fig. 5 ). The evolution of this technology into the orthopedic arena was enabled by the ability of the surgeon to obtain an accurate reproducible surgical result in all 6 degrees of freedom (3 translational and 3 rotational), which the human eye cannot reproduce to the same level of precision.
This system uses optical motion capture technology to dynamically track marker arrays fixed to the robotic arm, femur, and tibia, allowing the surgeon to freely adjust limb position and orientation during tactile-guided bone cutting. This robotic-assisted platform enables the surgeon to use a preoperative computer-assisted planning system and a high-speed burr controlled by a tactile guidance system intraoperatively, thus eliminating the need for conventional instrumentation.
There are several systems in place throughout the procedure to ensure accuracy is achieved. Tibial and femoral checkpoints must be verified before each section of bone is prepared. At any point throughout the procedure, the robotic-arm and mechanical burr tip can be removed from the surgical zone and a tracker probe can be used to visualize accuracy and cuts directly on the patient’s CT scan ( Fig. 6 ).
During burring, the surgeon receives visual, auditory, and haptic feedback to ensure adherence to the surgical plan. Visual feedback is depicted on the user interface module. Green bone on the screen indicates bone to be removed; white bone on the screen indicates the surgeon has removed the necessary bone, and red bone on the screen depicts when the planned implant placement threshold has been penetrated up to 0.5 mm ( Fig. 7 ). If penetration occurs beyond 0.5 mm, the burr tip shuts off. When approaching the implant boundaries, a surgeon receives auditory feedback with a beeping noise. Haptic feedback refers to the tactile sensation of the arm physically resisting a surgeon’s applied force when the haptic boundary is approached. Boundaries are specified to match the implant shape, size, and placement based on intraoperative planning. Cement tolerances are included in boundaries.
