Menisci, Lateral

We recruited subjects from the town of Framingham, Massachusetts. (The study cohort is distinct from the Framingham Heart Study and the Framingham Offspring Study cohorts.) Framingham census-tract data from the year 2000 and random-digit telephone dialing were used to recruit study participants; they were not selected on the basis of having knee or other joint problems. Repeated efforts were made to contact potential subjects by telephone, with a minimum of 15 attempts made over at least a 4-week period before a telephone number was retired from the list of randomly selected numbers. Potential subjects were called at least once each during daytime hours on a weekday, evening hours on a weekday, and weekend hours, although most were called several times during each of these periods. All persons who initially declined to participate were called back in an attempt to reverse their decision. Unless they were adamant at the time of the second refusal, they were also called a third time at least 2 weeks later. To enhance recruitment efforts, community leaders and Framingham senior centers were informed of the study, and flyers were posted in public areas. The institutional review board of Boston University Medical Center approved the study, including our call strategy, and we obtained written informed consent from all participants.
To be included in the study, subjects had to be at least 50 years of age and ambulatory (use of assistive devices such as canes and walkers was allowed), with no plans to move out of the area for at least 5 years. Subjects with a history of bilateral total knee replacement, rheumatoid arthritis,10 (link) dementia, or terminal cancer and those who had contraindications to MRI were excluded. Of 2582 people 50 years of age or older and living in Framingham who were contacted by random-digit dialing, 1830 expressed initial interest in participating in the study. Of these subjects, 1039 were examined during the period from October 2002 to June 2005. We assessed the integrity of the medial and lateral menisci in the right knee in persons who were willing and eligible to undergo MRI and whose scans were readable (991 subjects) (Fig. 1).

Cadaveric knee joints (n=11) were utilized for this Institutional Review Board (IRB)-approved study. Sample size was based on previously work³² reporting mean contact stress on the tibial plateau, which was the primary outcome measure in the present work. In the previous work, mean contact stress of 3.7 ± 0.4MPa was generated in response to an axial load of 1kN at full extension with the ACL intact. Thus, nine specimens were required to detect differences of 15% with 80% power and α = 0.05 using a repeated measures study design.
Fresh-frozen knees were thawed at room temperature 36 hours before testing. Average specimen age was 38 ± 12.5 ranging from 21 to 58 years old. Nine males and two females were utilized with six right legs and five left legs. Specimens were sectioned at the midshaft of the tibia-fibula and femur, leaving approximately 25cm of the shaft of each bone. The soft tissues surrounding the joint including capsular structures were left intact. Specimens were excluded if any gross joint abnormality, instability, or cartilage degeneration was observed visually through an anterior medial arthrotomy. A carpenter screw was drilled across the tibia and fibula proximally to stabilize the tibiofibular joint. The tibial and the femoral shafts were potted in bonding cement (Bondo/3M, Atlanta, Georgia). Two carpenter screws were drilled transversely in each shaft to ensure fixation between bone and cement.
The knees were loaded using a six-degrees-of-freedom robotic arm (ZX165U; Kawasaki) with ±0.3mm repeatability¹ (Fig. 1). A universal force-moment sensor (Delta; ATI, Apex, NC resolution: Fx = Fy = 1/8N, Fz = 1/4N, Tx = Ty = Tz = 10/1333Nm) mounted to the end of the robotic arm measured the forces acting across the knee joint. The potted femur was secured to a pedestal that was fixed to the floor. The tibia was attached to the robotic arm through a custom fixture. The specimen was aligned in full extension. Subsequently, anatomic landmarks were identified using a 3D digitizer (accuracy: 0.23mm) (MicroScribe; Immersion, San Jose, California) to define reference frames that describe motion of the tibia relative to the femur.
Rotations and translations of the tibia relative to the fixed femur were expressed using the convention described by Grood and Suntay^{15 (link)}. The medial and lateral femoral epicondyles defined the orientation of the flexion-extension axis. This axis was directed laterally and medially for right and left specimens, respectively. The bisection of this axis was assigned to be the origin of the femoral coordinate system. The long axis of the tibia was directed distally and defined internal-external rotation. Its orientation was defined by the most distal point on the center of the tibial shaft, and the bisection of the distal insertions of the medial and lateral collateral ligaments. The origin of the tibial coordinate system was assigned to be coincident with the origin of the femoral coordinate system at full extension. The common perpendicular of the flexion/extension axis and the internal/external rotation axis faced posteriorly and defined ad/abduction. Translations were expressed as the projection of the vector defined by the origins of the tibial and femoral coordinate systems onto each anatomic direction described above.
Force feedback algorithms were used to determine the position and orientation of the tibia that minimized the difference between the current and the targeted load to a resultant force ≤5N and a resultant moment ≤0.5Nm^{14 (link), 36 (link)}. Testing was begun by determining the path of passive flexion of the intact knee from full extension to 90° of flexion in 1° increments of flexion. To assess anterior stability, a 134N anterior force was applied at 0, 15, 30, 60 and 90° flexion. We tested at these angles because the posterolateral bundle of the ACL is the primary restraint to anterior forces at 0, 15 and 30° flexion, while the anteromedial bundle is the primary restraint to anterior forces at 60 and 90° flexion^{37 (link)}. To assess rotational stability combined moments of 8 and 4Nm in abduction and internal rotation, respectively, were applied at 5, 15 and 30° flexion^{20 (link)}. We tested at these angles because anterior translation is highest with ACL deficiency under these combined moments between full extension and 30° flexion^{20 (link)}. The position and orientation of the knee as found during the passive flexion path served as the starting points for the application of loads^{45 (link)}. Net knee motions in all directions were calculated for each loading condition, each flexion angle, and with the ACL intact, sectioned, and reconstructed. Net knee motion was defined as the change in knee position between the maximum applied load and the intial reference position along the path of passive flexion. The order of testing between the ACL deficient and reconstructed states was selected at random. ACL sectioning or reconstruction was performed through a medial parapatellar arthrotomy to allow direct visualization of the ACL anatomy. The arthrotomy was sutured closed after both procedures.
The native ACL was preconditioned by determining the motion required to achieve 134N anterior load at 30° flexion, and repeating this motion for ten cycles. Similarly, the medial collateral structures were preconditioned by determining the motion required to apply the combined moments at 15° flexion. This motion was then also repeated for ten cycles.
Single bundle ACL reconstruction was performed after resecting the native ACL by drilling in the center of the ACL footprints (Fig. 2)⁶ . A quadrupled semitendinosus and gracilis autograft measuring 9cm in length was prepared using an endobutton and 15mm loop. Graft material was harvested from each specimen and used only for that specimen. The diameter of the femoral tunnel was chosen to accommodate the size of the graft harvested from each specimen. The diameter of the tibial tunnel was drilled one mm larger then the femoral tunnel to account for increased graft diameter after suturing the tendons together. The femoral tunnel was made by first drilling a guide pin into the center of the native femoral ACL footprint through the medial parapatellar arthrotomy in a “medial portal equivalent” approach. It was drilled to a depth of 32mm. The Endobutton (Smith & Nephew, Inc., Andover, Massachusetts) drill bit was used to drill through the cortex. Adjustments to the tunnel depth were then made as needed. An ACL tibial drill guide set to 55° was positioned in the center of the tibial ACL footprint, adjacent to the anterior horn of the lateral meniscus. The graft was shuttled into position and the endobutton was deployed for femoral fixation. The knee was cycled twenty times. With the knee held in neutral rotation and 20° flexion^{44 (link)}, the sutures from the graft were tied around a cortical screw and washer, which had been placed in the tibia and fixed under 89N (20lbs) of pretension. A 10 × 25mm biointerference screw was placed in the tibial tunnel for supplemental fixation. Similar to the native ACL, the reconstructed ACL was preconditioned by determining the motion required to achieve 134N anterior translation at 30° flexion, and then cycling ten times.
After determining knee motions for each loading condition and each state of the ACL, a stress transducer (4010N, Tekscan, South Boston, Massachusetts) was slid beneath the menisci and sutured in place so that it remained fixed to the tibial plateau (Fig. 3). All kinematic pathways were replayed, and the contact stresses in the medial and lateral compartments of the tibia were recorded. To assess spatial variation in contact stress patterns on the tibial plateau, the area of the stress transducer was divided into six sectors in each compartment (anterior, middle, or posterior in anterior-posterior direction, and central or peripheral in medial-lateral direction) (Fig. 3). The mean contact stress in each sector was calculated at the position corresponding to the maximum applied external load for each loading condition. Mean contact stress was chosen as a representative measure for the distribution of load at the articulating surface.
The stress transducer was calibrated prior to testing by loading it to 20% and 80% of the maximum expected load and then fitting these data with a two-parameter power function. The calibration accuracy was tested by loading the sensor in an MTS loading system (MTS Systems, Eden Prairie, Minnesota) with a Instron Controller (8500; Instron, Norwood, Massachusetts) and a 444.8N (100-lb) load cell (Interface, Scottsdale, Arizona) after calibration. Repeatability of sensor measurements over the course of testing was assessed by repeating a subset of motions (n=14) in a subset of specimens (n=6). These data were presented as mean and standard deviation of the percent change in the total force measured by the sensor at the maximum applied external load across the repeated measurements.
Mean contact stress was compared across ACL intact, deficient and reconstructed conditions on a sector-by-sector basis using generalized estimating equations (GEE)^{16 (link)}. This technique is suitable for data that are not normally distributed. A separate analysis was performed for each applied load at each flexion angle. Similarly, ML and AP translations, and axial rotation, and ab/adduction were compared across each condition of the ACL, at each applied load, and at each flexion angle using GEE. Statistical significance for all comparisons was set at p<0.05. Means, standard deviations and 95% confidence intervals were calculated for all outcomes.
Associations between kinematics and sectors where mean contact stress remained abnormal following ACL reconstruction were assessed using multiple linear regression. The differences between the intact and ACL reconstructed conditions were used in this regression model for both kinematic and contact stress measures. Regression coefficients with p<0.05 were reported along with their 95% confidence intervals and the coefficient of determination (r^{2 (link)}).