Abstract
Remarkable strides made in medical technology and techniques of total knee arthroplasty over past 5 years. These changes have included: minimally invasive surgical techniques, pain management, navigation, kinematic design of prosthesis and recently custom fitted surgical guides based on the anatomic axis. To date, there has been little documentation of the use of these custom-cutting surgical guides.
There has been significant controversy as to the necessity of using the neutral alignment of the mechanical axis for this surgery for a long lived replacement. A recent study by Pagnano et al in 2008 demonstrated that it could not be confirmed that improvement in the mechanical axis to zero would lead to a long-term improvement in survivorship, and it was noted that there was actually a slight trend for the outliers to be more successful. A recent study (Three-Dimensional Morphology and Kinematics of the Distal Part of the Femur Viewed in Virtual Reality Eckhoff et al, JBJS 2005) provides kinematic and morphologic validation for a single cylindrical flexion-extension axis of the knee. This fixed flexion-extension axis is best approximated by the axis of cylinders, fit to the circular posterior femoral condyles, and is designated the cylindrical axis of the knee.
An innovative surgical technique of total knee arthroplasty has been developed using MRI-based custom fitting cutting blocks. This technique advocates the use of an individual knee MRI, utilizes the cylindrical axis and proceeds with precise measurements of the arthritic knee. Proprietary software creates a 3-dimensional model of the knee and then corrects the deformity virtually, and recreates the knee's pre-arthritic alignment. Guides are designed to fit on diseased bone and set transverse resection and rotation and enable implant placement that restores joint to pre-disease position.
32 patients were enrolled in this IRB-approved study of total knee replacement. Pre-operative standing anterior-posterior lower extremity x-rays were required for assessment of the degree of malalignment. Patients with a malalignment greater than 15 degrees were excluded from the study. Only 26 knees with varus alignment were in the final study group since the valgus group was very small in number. Computer navigation appears to provide the most precise kinematic measurement of the knee, and was used during the operation to assess and quantitate the pre-operative, intra-operative, and post-operative alignment and potential correction. The pre-operative pathologic malalignment was documented by navigation and the post-operative alignment did demonstrate some correction of this malalignment back to the presumed pre-arthritic alignment.
Change in alignment of 26 varus knees was documented as the following: Pre-op AP standing XRay: average 6.9 degrees varus; Pre-op Navigation: average 6.3 degrees varus; Post-op Navigation: average 3.4 varus degrees. This resulted in post-operative correction of the varus knee to 2.9 degrees.
Documentation of resections planes was noted as the following: Femur AP Resection 3.0 degrees valgus (r: 3.5 varus-4.0 valgus); Femur Distal Resection: 3.7 degrees flexion (r: 2.5 ext-10.0 flex); Femur Rotation Resection: 3.6 degrees internal rotation (r: 2.5 ext-7.5int); Tibia AP Resection 3.3 degrees varus (r: 2.0 valgus-6.0 varus); Tibia Slope Resection: 3.7 degrees posterior (r: 0.5 ant-9.0 post).
This study did support the premise that custom-fitting surgical guides locate the cylindrical axis, as determined by Eckhoff et al. This may provide the patient with less soft tissue stress and allowing a quicker return to function as reported in earlier studies. This surgeon did recognize obstacles using the custom-fitting surgical guides including determining the extent of debridement of soft tissue and osteophytes to allow appropriate capture of the blocks, as well as the risk of PCL injury. Navigation can be used as a training tool to aid in the prevention of significant error.
By locating the cylindrical axis, the natural kinematics of the knee are addressed, including the soft tissue tension. As the mechanical axis is being challenged, we look to the cylindrical axis as our potential objective, unique for each patient. Further validated studies are required, to understand the operative kinematics and the long term effects of the cylindrical axis.
