Despite the success of total knee arthroplasty (TKA) restoration of normal function is often not achieved. Soft-tissue balance is a major factor leading to poor outcomes including malalignment, instability, excessive wear, and subluxation. Mechanical ligament balancers only measure the joint space in full extension and at 90° flexion. This study uses a novel electronic ligament balancer to measure the ligament balance in normal knees and in knees after TKA to determine the impact on passive and active kinematics. Fresh-frozen cadaver legs (N = 6) were obtained. A standard cruciate-retaining TKA was performed using measured resection approach and computer navigation (Stryker Navigation, Kalamazoo, MI). Ligament balance was measured using a novel electronic balancer (Fig 1, XO1, XpandOrtho, Inc, La Jolla, CA, USA). The XO1 balancer generates controlled femorotibial distraction of up to 120N. The balancer only requires a tibial cut and can be used before or after femoral cuts, or after trial implants have been mounted. The balancer monitors the distraction gap and the medial and lateral gaps in real time, and graphically displays gap measurements over the entire range of knee flexion. Gap measurements can be monitored during soft-tissue releases without removing the balancer. Knee kinematics were measured during active knee extension (Oxford knee rig) and during passive knee extension under varus and valgus external moment of 10Nm in a passive test rig. Sequence of testing and measurement:
Ligament balance was recorded with the XO1 balancer after the tibial cut, after measured resection of the femur, and after soft-tissue release and/or bone resection to balance flexion-extension and mediolateral gaps. Passive and active kinematics were measured in the normal knee before TKA, after measured resection TKA, and after soft-tissue release and/or bone resection to balance flexion-extension and mediolateral gaps.Background
Methods
Despite the success of total knee arthroplasty (TKA) restoration of normal function is often not achieved. Soft tissue balance is a major factor for poor outcomes including malalignment, instability, excessive wear, and subluxation. Computer navigation and robotic-assisted systems have increased the accuracy of prosthetic component placement. On the other hand, soft tissue balancing remains an art, relying on a qualitative feel for the balance of the knee, and is developed over years of practice Several instruments are available to assist surgeons in estimating soft tissue balance. However, mechanical devices only measure the joint space in full extension and at 90° flexion. Further, because of lack of comprehensive characterization of the ligament balance of healthy knees, surgeons do not have quantitative guidelines relating the stability of an implanted to that of the normal knee. This study measures the ligament balance of normal knees and tests the accuracy of two mechanical distraction instruments and an electronic distraction instrument. Cadaver specimens were mounted on a custom knee rig and on the AMTI VIVO which replicated passive kinematics. A six-axis load cell and an infrared tracking system was used to document the kinematics and the forces acting on the knee. Dynamic knee laxity was measured under 10Nm of varus/valgus moment, 10Nm of axial rotational moment, and 200N of AP shear. Measurements were repeated after transecting the anterior cruciate ligament, after TKA, and after transecting the posterior cruciate ligament. The accuracy and reproducibility of two mechanical and one electronic distraction device was measured.Background
Methods
Kinematic studies are used to evaluate function and efficacy of various implant designs. Given the large variation between subjects, matched pairs are ideal when comparing competing designs. It is logical to deduce that both limbs in a subject will behave identically during a given motion [1], barring unilateral underlying pathology, thus allowing for the most direct comparison of two designs. It is our goal to determine if this is a valid assumption by assessing whether or not there are significant differences present in the kinematics of left and right knees from the same subject. Gait studies have compared pre-and postoperative implantation kinematics for various pathologies like ACL rupture [2] and osteoarthritis [3, 4]. We designed a study to assess squatting in cadaver specimens. Sixteen matched pairs of fresh-frozen cadavers, (Eleven males, five females; aged 71 years [± 10 yrs]) were tested. Each knee, intact, was tested by mounting it on a dynamic, quadriceps-driven, closed-kinetic-chain Oxford knee rig (OKR), which simulated a deep knee bend from full extension to 120° flexion. We chose femoral rollback, tibiofemoral external rotation, tibial adduction, patellofemoral tilt and shift as our outcomes, which were recorded using an active infrared tracking system.Introduction:
Methods:
Despite over 95% long-term survivorship of TKA, 14–39% of patients express dissatisfaction due to anterior knee pain, mid-flexion instability, reduction in range of flexion, and incomplete return of function. Changing demographics with higher expectations are leading to renewed interest in patient-specific designs with the goal of restoring of normal kinematics. Improved imaging and image-processing technology coupled with rapid prototyping allow manufacturing of patient-specific cutting guides with individualized femoral and tibial components with articulating surfaces that maximize bony coverage and more closely approximate the natural anatomy. We hypothesized that restoring the articular surface and maintaining medial and lateral condylar offset of the implanted knee to that of the joint before implantation would restore normal knee kinematics. To test this hypothesis we recorded kinematics of patient-specific prostheses implanted using patient-specific cutting guides. Preoperative CT scans were obtained from nine matched pairs of human cadaveric knees. One of each pair was randomly assigned to one of two groups: one group implanted with a standard off-the-shelf posterior cruciate-retaining design using standard cutting guides based on intramedullary alignment; the contralateral knee implanted with patient-specific implants using patient-specific cutting guides, both manufactured from the preoperative CT scans. Each knee was tested preoperatively as an intact, normal knee, by mounting the knee on a dynamic, quadriceps-driven, closed-kinetic-chain Oxford knee rig (OKR), simulating a deep knee bend from 0° to 120° flexion. Following implantation with either the standard or patient-specific implant, knees were mounted on the OKR and retested. Femoral rollback, tibiofemoral rotation, tibial adduction, patellofemoral tilt and shift were recorded using an active infrared tracking system.Introduction:
Methods:
Dislocation remains a major early complication after total hip arthroplasty (THA), and range of motion (ROM) before impingement is important in joint stability. Factors contributing to dislocation include design specific factors such as head-neck ratio, surgeon-related factors such as component placement, and patient-related factors such as bony anatomy. To study the relative importance of these factors, we analysed the effects of patient anatomy, implant design, and component orientation on hip ROM. Femoral and acetabular geometry were extracted from CT scans of 20 hips. CAD models of four different THA component designs were virtually implanted in the 3D-CT reconstructed anatomic models. The major design differences were in head-neck ratio and neck-stem angle. A previously reported contact detection model (D’Lima, J Orthop Research 2008) was used to measure restriction in hip ROM due to prosthetic or bony impingement. The following patient parameters were measured on plain AP radiographs: acetabular inclination, acetabular depth ratio, the arc-length between the tip of greater trochanter and ilium, and the arc-length between lesser trochanter and ischium. Multiple linear regression was used to determine correlation between radiographic parameters and hip ROM in flexion, extension, adduction, abduction, and external rotation. Mean head size was 51 ± 2mm, mean anatomic acetabular inclination was 41° ± 2, and mean acetabular depth ratio was 460 ± 60. When the cup and stem were implanted for best fit to the anatomy, mean hip ROM was 125° ± 8 (flexion), 57° ± 17 (extension), 29° ± 13 (adduction), 69° ± 7 (abduction), and 42° ± 13 (external rotation). Implanting the cup in “optimal” surgical alignment of 45° abduction and 20° anteversion reduced mean hip flexion, extension and abduction and increased adduction. Subject-to-subject variation was substantially greater than variation between CAD designs (differences in head-neck ratio) or component orientation (between ideal and anatomic). Hip flexion correlated moderately with acetabular abduction angle and the angle of the flare of the iliac wing (R2 = 0.59, p = 0.03). Hip abduction correlated moderately with the angle of the flare of the iliac wing and the length of the arc from the tip of the greater trochanter to the ilium (R2 = 0.50, p = 0.05). A universal cup position that permits optimal range of motion in all patients may not be valid. Since patient-related factors overshadowed implant design, cup position should be tailored to the individual patient. Preoperative radiographs can help predict postoperative hip ROM although not as accurately as 3D-CT reconstructions. These results may lead to enhancements in surgical navigation techniques.
