Using single-plane flat-panel imaging, we demonstrated non-invasively that normal in vivo patellofemoral kinematics are tightly coupled with femorotibial kinematics. When the daily activities were assessed collectively, we observed patellar flexion angle increased and the patella translated distally as the femorotibial joint flexed. The highest demand activity, trotting, produced kinematic patterns that did not follow the same pathways seen during walking and stand-to-sit motions. Although a portion of the patella was positioned distal to the trochlear groove during deep flexion while sitting, the base of the patella always remained positioned within the femoral trochlea during the activities analyzed in these dogs.
Our results are similar to the findings from a canine cadaveric investigation, in which passive patellofemoral joint motion was described over a smaller range of femorotibial motion . Both studies demonstrated that all three kinematic parameters were linearly related to the femorotibial flexion angle. Our results, however, differ slightly with respect to the magnitude of observed change; for example, changes in femorotibial flexion angle from 90 to 150° induced proximodistal patella translation of approximately 13 mm in the cadaver study versus approximately 18 mm in our in vivo study . The change in patellar flexion of approximately 25° was reported in the cadaver study, whereas we found a change of 44° over the equivalent femorotibial range of motion . There are several explanations for the discrepancies between the studies. Most obviously, our investigation was an in vivo dynamic analysis, accounting for all the complex forces acting on the patella in vivo. Anatomic differences between breeds may have been a factor, as our study used Labrador Retrievers while the cadaver study used mix-breed dogs. Equally importantly, variations in coordinate assignation and reference points have been shown to dramatically affect patellar tracking patterns in humans, even within the same individual . To the author’s knowledge, this is only the second report characterizing patellar kinematics in dogs. Standardized coordinate systems and reference points for future studies would allow more meaningful comparison between investigations.
The most significant finding of the study was that the relationship between patellar poses and femorotibial flexion angle varied according to the phase of the gait cycle during trotting; the patella was positioned more proximal, more cranial, and more flexed in early swing phase when compared to late swing phase at the identical femorotibial flexion angle (Fig. 6). This offset likely contributed to the lack of a statistical linear correlation with the femorotibial flexion angle. The cause of this offset upon entering and exiting swing during trotting remains to be clarified. Potential causes include the varying magnitude at which the pelvic limb musculature, particularly the quadriceps muscle group, are contracting and acting on the patella, as well as the secondary motions of the tibia such as internal-external rotation and craniocaudal tibial translation. While an increased force of quadriceps contraction did not have significant effect on patellar movement in a human cadaveric study, an in vivo MRI study demonstrated the resting patellar position in the trochlea groove could be altered by muscular contraction during either open- or closed-chain exercises [18, 19]. Surface electromyographic studies in dogs have demonstrated widely varying patterns of vastus lateralis contraction according to differing activities, which could alter patellofemoral poses during different phases of the gait cycle as observed in our study . Electromyographic studies performed concurrently with radiographic kinematic analyses might improve our understanding of how muscular contraction alters patellar kinematics in dogs.
The single-plane flat-panel imaging and shape-matching methodology utilized in this study was previously validated for femorotibial kinematics in dogs [15, 21]. The precision of this methodology for this joint was determined to be within 1.28 mm and 1.58° with an intraobserver variability of less than 0.52 mm and 0.91° for translation and rotations, respectively . These values reported for the femorotibial joint are unlikely to be directly applicable for the patellofemoral joint due to differences in bone geometry. Studies performed in humans using similar methodology to evaluate the patellofemoral joint kinematics reported accuracy within 0.6° for in-plane rotations and 1.5 mm for in-plane translations . Analysis in this study was confined to sagittal plane translations and rotation due to the uniplanar nature of the technique and the patellofemoral joint anatomy. Although studies have not been performed to specifically validate the accuracy of our methodology for determining patellofemoral joint kinematics in dogs, a pilot series assessing repeatability of 3-D to 2-D image registration for this joint appeared to consistent with the results observed in other studies [23, 24].
The 3-D patella model was approximated to remain central within the trochlear groove such that the center of the patella’s articulating surface remained as congruent as possible with the trochlear groove in the axial plane. We utilized the 3-D geometry of the trochlear groove to define medial-lateral translation as well as patellar tilt. The kinematic parameters in the sagittal plane may have been different if the assumptions were not made. Studies in humans have shown the axial plane topography of the trochlear groove could be used to predict axial plane patellar kinematics, supporting our methodology . However, coronal plane rotation, conventionally known as patellar rotation, was not readily predictable by trochlear anatomy in humans . Bi-plane radiographic analysis would be required to gain a more thorough intricate understanding of canine patellofemoral joint motion in vivo.
Our data should be useful as a baseline of normal patellofemoral kinematics in dogs, against which comparisons can be made in future studies. The main goal of this study was to characterize normal patellofemoral motion in order to define the change in kinematics caused by cranial cruciate ligament rupture in a future study by our group. Because the most profound femorotibial kinematic abnormalities with cranial cruciate ligament rupture occur in the sagittal plane, it is logical to expect that the patellofemoral kinematics we reported, including patellar flexion, proximodistal translation, and craniocaudal translation, may be disrupted by the condition. Indeed, cadaveric studies have found the cranial cruciate ligament rupture can alter patella alignment and patellofemoral contact mechanics . Patellofemoral mechanics are also of particular interest with the surgical treatment of cranial cruciate ligament rupture as stifle extensor mechanism abnormalities frequently occur following various procedures used to address the disease . Our results are likely less applicable to patellar luxation, where the major abnormalities in motion are in the coronal plane.