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System supports functional, patient-specific THR component alignment
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Edge-loading, accelerated wear, impingement and dislocation are leading contributors to total hip replacement revision. Component malposition is a major contributor to these variables, which result in an increase in revision THR surgery. Defining an intraoperative target alignment is highly complex and the accuracy of positioning the acetabulum within a “safe zone,” as described by Lewinnek and colleagues, is highly variable. Computer-assisted orthopedic surgery and, more recently, robotics were introduced to improve surgical precision, but there has been slow acceptance by the orthopedic community. This is likely due to the poor definition of what constitutes the correct target alignment for a given individual. Recent literature has questioned whether the commonly accepted guidelines for implant alignment should be considered ideal for every patient since more failures have been observed when following these historical recommendation, than when not.
Instability and edge-loading occurs during functional activities when the position of the pelvis and femur are different from that seen on standard radiographs or intraoperatively by fluoroscopy. It is universally agreed that hip kinematics are specific to an individual and, consequently, component alignment should be planned using dynamic information to minimize the risk of failure.
The Optimized Positioning System or OPS (Corin Group) is a system for patient-specific preoperative planning, intraoperative delivery and postoperative analysis. It consists of a preoperative planning and analysis component, along with patient-specific instrumentation. The planning uses standard radiographs to assess each patient’s alignment, bone morphology and kinematics. A software program is used to analyze the bearing contact mechanics and impingement using a rigid-body dynamic simulation of functional activities.
Source: Herrick J. Siegel, MD
Step 1: Preoperative imaging
A few weeks before the procedure, each patient has three lateral functional radiographs taken: standing, flexed seated and step-up with the contralateral leg raised. The functional images are then loaded into a software program which records data regarding pelvic tilt, sacral slope and lumbar lordotic angles. Using the measured angles, the positions of the bones at the limits of hip flexion and extension are defined. In addition, the patient’s bony geometry is viewed using a low-dose CT scan. The 3-D coordinates of soft tissue and bony landmarks are identified and recorded.
Step 2: Implant dynamic simulation
Using the Corin 3-D acetabular shell geometry, an engineer then performs preliminary templating during which the acetabular implants are virtually positioned within the patient’s acetabulum in such a way that the anatomy is restored. The surgeon can then interact and change the preoperative plan according to any patient-specific clinical observations or requirements, such as preoperative leg-length discrepancies or deformities.
The segmented bone models and templated component alignment are then input into a rigid-body dynamic simulation of flexion and extension activities, driven by kinematic input from the functional radiographs. The computer simulation calculates the magnitude and direction of the hip joint reaction force throughout the two activities — hip flexion and extension — and determines the path of the contact patch as it traces across the articulating surface. The contact patch paths are then reviewed by the surgeon in the form of a polar plot that represents the bearing surface in two dimensions as viewed perpendicular to the face of the cup. The polar plots are generated for nine different cup orientations, which are defined by angles of radiographic inclination and anteversion. The resultant nine plots demonstrate the effect of cup orientation on contact mechanics across a patient-specific zone, which assists the surgeon in determining the optimal cup orientation for the patient.
Step 3: Report analysis, finalized plan
The preoperative plan, including results from the dynamic analysis and implant templating, is presented to the surgeon for approval prior to surgery. The system determines a preliminary target cup orientation based on a series of preferences defined by the surgeon, which takes into consideration the surgeon’s accepted ranges for acetabular inclination and anteversion, surgical approach, acetabular shell coverage, acceptable boundaries from the anterior and posterior edges of the bearing, as well as any expected changes in pelvic kinematics postoperatively.
The surgeon may change the templated implants and target orientation prior to finalizing the plan.
Step 4: Intraoperative guides
The resultant acetabular guide is designed to fit within the patient’s acetabulum and guide the planned cup orientation. The guide and corresponding bone models are 3-D-printed from medical grade nylon and sterilized (Figure 1, page 4). The models have inner markings that assist with alignment of the guide within the patient’s acetabulum (Figures 2 and 3). The guides can be used with any THR surgical approach, including direct anterior, lateral or posterior.
After the surgeon performs his or her selected exposure, the femoral neck osteotomy is made per the surgeon’s preoperative template. The acetabular guide is then seated within the acetabulum after the fat pad and any soft tissue in the acetabular fossa are excised (Figure 4). The in vivo position can then be checked against the markings on the sterile bone model. A laser handle connects to the axis of the guide and projects the target orientation onto the OR ceiling or wall (Figure 5). A second laser mounted to the pelvis is orientated to converge with the projection on the ceiling or wall and serves as a comparison control for laser alignment. Alternatively, an external laser may be mounted to a marker screw in the operative field (Figure 6, page 4). Any intraoperative movement of the pelvis will therefore not affect the target orientation, which is not dependent on being in any particular position in the OR. With the guide in place, the laser handle is then placed on the guide and pointed at the OR wall. The alignment of the pelvic laser is adjusted to converge with the acetabular guide laser as projected on the wall or ceiling and the dial is secured. A mark is placed on the wall where the lasers converge. It serves as the target location during impaction of the final implant. The surgeon should be focused on the OR during laser alignment and must be comfortable with visualization of the marker as it is, because the marker is used later to reference alignment.
Reaming is completed using the surgeon’s usual technique and is done to the depth that was planned preoperatively. Final cup orientation is guided by a laser located on the end of the impactor handle. The handle should be orientated so the laser on the handle aligns with the projection of the control reference laser. Cup orientation may also be confirmed by referencing anatomical features, such as osteophytes around the acetabulum referenced to the rim of the cup, and using the markings on the bone model. The placement of the patient-specific guide and planned cup in the acetabulum can also be visualized in 3-D models on a tablet computer during the surgery, if preferred. Upon fully seating the acetabular cup, the liner is impacted into the cup, after which the THR procedure may proceed as it would during the surgeon’s usual cases.
Most orthopedic surgeons find this technique adds little time to the surgery. It is straightforward, reproducible and encourages the surgeon to think about spinopelvic mobility and individual variations in patient anatomy.
Role of pelvic tilt
The Cover Story in the August 2017 issue of Orthopedics Today, “Aging spine poses challenges for acetabular cup positioning,” addresses a crucial issue that impacts acetabular cup positioning related to the flexibility of the spine. When the lumbar spine is fused for treatment of degeneration or deformity, it impacts the pelvic tilt (PT), and that changes the position of the acetabular cup. It also changes the amount of PT change between sitting and standing. In the article, Aaron J. Buckland, MD, noted these changes may affect the patient’s risk for dislocation.
In the same article, Douglas A. Dennis, MD, at the University of Denver and attending surgeon at Colorado Joint Replacement, said, “If a fusion involves the lumbar spine, particularly a fusion to the sacrum, that can have a significant effect on lumbopelvic mobility, which then can change the 3-D position of where [a patient’s] pelvis is positioned in daily activity.”
OPS may not only have a significant impact on improving instability in patients with spinopelvic immobility, but it can also reduce edge-loading and impingement in patients undergoing THR. The positioning system may lessen concerns about ceramic-on-ceramic THR articulations.
To date, more than 3,000 patients in Australia and Europe have undergone this type of preoperative THR planning using this FDA-cleared technology, which helps surgeons achieve functional, patient-specific component alignment in THR.
- References:
- Abdel MP, et al. Clin Orthop Relat Res; 2016; doi:10.1007/s11999-015-4432-5.
- Buckland AJ et al. J Bone Joint Surg Am. 2015. doi:10.2106/JBJS.O.00276.
- Lewinnek GE, et al. J Bone Joint Surg Am. 1978;60:217-220.
- Stefl M, et al. J Bone Joint Surg. 2017. doi:10.1302/0301-620X.99B1.BJJ-2016-0415.R1.
- For more information:
- Herrick J. Siegel, MD, can be reached at University of Alabama at Birmingham, Department of Orthopaedic Surgery, Division of Adult Reconstruction & Orthopaedic Oncology, 1313 13th St. South, Birmingham, AL 35205; email: hsiegel@uabmc.edu.
Disclosure: Siegel reports he is a paid consultant and speaker for Corin Orthopaedics, but does not receive financial support related to the technology discussed.