Across the knees, the medial and lateral quadriceps components shifted the patella in rather different directions (Fig. 6). At full knee extension, contraction of the VMO mainly pulled the patella medially with the corresponding DOTx = 0.860 ± 0.098 (mean ± standard error (SE)) significantly greater than DOTy = 0.460 ± 0.178 (proximal shift, P < 0.002) and DOTz = 0.222 ± 0.192 (anterior shift, P < 0.001) (Fig. 6). With the knee flexed at 20°, VMO contraction still mainly pulled the patella medially, but the minor shifts changed to the posterior and distal directions.
Different from the VMO that pulled the patella medially, the VML pulled the patella more in the proximal (P < 0.005) and anterior (P < 0.0001) directions than in the medial direction. At full knee extension, DOT for the VML were DOTx = 0.427 ± 0.126 (medial shift), DOTy = 0.904 ± 0.081 (proximal shift), and DOTz = −0.008 ± 0.206 (Fig. 6). With the knee flexed to 20°, medial and proximal shifts were the major motion induced by VML, with DOTx = 0.708 ± 0.167, DOTy = 0.701 ± 0.210, and DOTz = −0.085 ± 0.245 (Fig. 6).
Selective contraction of the VL mainly moved the patella proximally, plus moderate lateral pull with DOTx = −0.337 ± 0.089, DOTy = 0.938 ± 0.055, DOTz = −0.081 ± 0.161 at full knee extension (Fig. 6). Among the three linear translations, the quadriceps components generated little patellar movement in the anterior-posterior direction, except for the VL at 20° knee flexion, in which the patella was pulled by the VL posteriorly with DOTx = 0.293 ± 0.163, DOTy = 0.612 ± 0.194 and DOTz = −0.735 ± 0.176 (Fig. 6).
The medial and lateral quadriceps components rotated the patella rather differently about the mediolateral tilt and mediolateral rotation axes but similarly in extension (Fig. 7). The extension component was significantly larger than the medial rotation component for both VMO (P < 0.005) and VML (P < 0.02).
Medial tilt was the major patellar rotation induced by VMO contraction at full knee extension and the corresponding DOR2 at the neutral tibial rotation was 0.814 ± 0.208 (Fig. 7), which was significantly larger than DOR3 = −0.228 ± 0.154 (corresponding to lateral rotation, P < 0.0005). On the other hand, the VL generated little lateral tilt with DOR2 = −0.281 ± 0.273 at full knee extension.
The VMO generated little patellar mediolateral rotation, whereas medial rotation was the major motion generated by the VL (Fig. 7). The corresponding DOR3 at full knee extension was −0.228 ± 0.154, −0.438 ± 0.217, and 0.757 ± 0.214 for the VMO, VML, and VL, respectively (Fig. 7).
The amplitude of patellar movement induced by comparable quadriceps contractions was generally reduced from full knee extension to the flexed position, with significant decrease in medial tilt, medial and proximal shifts of VMO (P < 0.05), proximal shift of VL (P < 0.0002), and all the DOFs of VML (P < 0.05). On the other hand, the DOR and DOT might change as the knee flexed, dependent on the specific quadriceps components. When the knee position was changed from full extension to 20° flexion, only the VMO changed its main rotation component from patellar extension (DOR1 = −0.534 ± 0.237) to flexion (DOR1 = 0.846 ± 0.554). The major patellar movement induced by VML and VL contraction at flexed knee did not change from that at full knee extension (Figs. 6 and 7). VML kept its main actions of patellar extension, medial tilt (DOR1 = −0.686 ± 0.289, DOR2 = 0.568 ± 0.340, DOR3 = −0.455 ± 0.240, Fig. 7), and proximal and medial shifts (DOTx = 0.708 ± 0.167, DOTy = 0.701 ± 0.210, DOTz = −0.085 ± 0.245, Fig. 6). With the knee flexed, VL maintained the main function of medially rotating and proximally shifting the patella, and generated less patellar extension/flexion and less proximal but more posterior shifts with DOR1 = −0.035 ± 0.295, DOR2 = 0.088 ± 0.374, DOR3 = 0.995 ± 0.308, DOTx = 0.293 ± 0.163, DOTy = 0.612 ± 0.194, and DOTz = −0.735 ± 0.176 (Figs. 6 and 7). With the knee flexed, VMO generated less medial rotation than VML (P < 0.005) and VL (P < 0.02).
Corroboration of the in vivo and noninvasive patellar tracking.
First, in the cadaver model, patellar tracking measured using markers attached to the patellar clamp and to metal screws inserted into the patella, tibia, and femur in a triad formation matched closely, especially in patellar translations (Fig. 8). Over six trials, compared with the measurements with the pin inserted into patella, the patellar movements measured with the patellar clamp gave errors of −0.38 ± 0.14°, 0.05 ± 0.08°, −0.03 ± 0.32°, 0.02 ± 0.04 mm, −0.26 ± 0.04 mm, and −0.01 ± 0.05 mm (mean ± SE) for flexion, medial tilt, medial rotation, medial shift, proximal shift, and anterior shift, respectively. Second, fluoroscopic imaging on human subjects also showed that the patellar clamp closely followed the patellar movement under selectively activation of individual heads of the quadriceps.
Although patellar tracking is involved in most knee functional activities and abnormal patellar tracking may be involved in various pathological conditions, there is a lack of information on three-dimensional patellar tracking induced by individual quadriceps components, especially under in vivo and noninvasive conditions. The present study provided us an in vivo and noninvasive tool to evaluate three-dimensional patellar tracking induced by selective activation of an individual quadriceps component. Experimentally, the in vivo approach used in this study was corroborated with in vitro and fluoroscopic methods. It was found that each quadriceps component moved the patella in its unique way, which was different among the different quadriceps components. It was found that the medial and lateral quadriceps components moved the patella in rather different directions with the VM mainly pulled the patella in the medial and proximal direction and VL pulling more proximally than laterally. Within the VM, the VMO pulled patella more medially and the VML more proximally. The medial and lateral quadriceps components rotated the patella rather differently about the mediolateral tilt and mediolateral rotation axes but similarly in patellar extension. Medial tilt was the major patellar rotation induced by VMO contraction at full knee extension. With the knee at the more flexed positions, the amplitude of patellar movement induced by the same quadriceps contractions was reduced from those at full knee extension, and VMO changed its major action from extending to flexing the patella.
In comparable cases, patellar tracking presented in this study was in general consistent with previous in vitro and in vivo studies. Little information has been provided for the contribution to the patellar movement by the individual head of quadriceps except for the VMO, largely because of the belief of its important role in preventing patellar mal-tracking (3,8,13,24). Our results showed VMO actions (pulling and tilting the patella medially) similar to that reported in the literature (3,22,24). These major actions change with knee flexion angle substantially, mainly in patellar extension and medial tilt. It was shown in our study that when knee flexion was changed from 20° to full extension, patellar medial tilt became the major function of VMO, whereas it was the smallest rotation in the more flexed knee. It implies that the VMO’s major role, especially during the last stage of knee extension when the patellar has little bony constraint by the trochlear grove, was to counterbalance the pull of the lateral component of the quadriceps, probably attributed to its large insertion angle onto the patella (1,16,27). At the same time, VMO also changed its role from flexing to extending the patella. This was consistent with what Koh et al. (13) found in their in vivo patellar tracking measurement when the VMO was activated by electrical stimulation. In several studies focused on the anatomy of VMO (10,20,21,30), it was reported that the two portions of the vastus medialis have marked difference in fiber alignment, with VMO generally directs much more medially (45–65° from the femoral axis) and VML only has about a 15–25°angle with the femoral axis. Besides, Raimondo et al. (21) suggested that the insertion angle of VMO onto the patella may be related to patellar location which is related to knee flexion. When the knee is in a more extended position, the patella seats on the superior part of trochlear grove of femur and thus results a larger insertion angle of VMO onto the patella. As a result, contraction of the VMO pulls the patella more in the direction of medial tilt and medial shift. On the other hand, VMO contributed the least to the patellar medial rotation among all three muscle heads, indicating the line of action of the muscle is very close to the rotation center of the patella.
Although there are different opinions about whether VMO should anatomically be defined as a separated muscle itself, it is generally agreed that the two portions of the vastus medialis function differently. As shown above, the main actions of the VML was to extend and proximally shift the patella. Furthermore, unlike the actions of the VMO, VML was not influenced much by knee flexion. This may be attributed to its small insertion angle onto the patella, which makes it pull along the axis of the femur, despite the change of knee flexion angle.
The interesting result that the VL rotated the patella medially in some of the subjects was corroborated in our experiment by visual inspection and videotaping. There were anatomical findings (7,30) suggested different portions of VL inserted on different parts of the patella with different insertion angle, creating variations in the force applied to the patella. Sakai et al. (23) also briefly demonstrated the possibility that the VL might exert medial rotation moment around the center of patella.
Most patellar tracking studies were done in vitro using cadaver knees; much less research has been done in vivo. Koh et al. (13) evaluated patellar tracking on one human subject with reflective markers placed on intracortical pins inserted into the patellar and femur. Their finding that VMO contraction induced patellar extension at fully extended knee but generated patellar flexion in more flexed knee (13) was confirmed by our results in this study (Fig. 7).
To evaluate the selective activation of individual quadriceps components through surface stimulation, patellar tracking was also measured in selected subjects together with the compound muscle action potential (M-wave) in the several quadriceps components. M-wave was only observed in the quadriceps component that was stimulated, indicating selective activation of the stimulated component and negligible co-contraction of other quadriceps components (Fig. 9).
A limitation of the present noninvasive study was that the patellar clamp could only be used at a small range of motion, approximately from 20° flexion to full knee extension. If more flexed knee positions need to be investigated, other approaches need to be used. Fortunately, the more extended knee positions happen to be the range where the patellar malalignment and abnormal tracking tend to be more severe. Furthermore, the more extended knee positions are also in the range where it becomes difficult to obtain the standard tangential (axial) view patellar radiograph because it is technically difficult to see the patella on the radiograph at the more extended knee positions (2,4,11,25).
The authors acknowledge the support of the National Institutes of Health (AR45634).
There are no professional and financial relationships between the authors and companies and manufactures. The results of the present study do not constitute endorsement of any product by the authors or ACSM.
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Keywords:©2004The American College of Sports Medicine
PATELLA; KINEMATICS; PATELLAR MALALIGNMENT; KNEE