BIOMECHANICS OF THE PATELLOFEMORAL JOINT: A
CLINICAL OVERVIEW
James B. Stiehl, MD.
Orthopedic Hospital of Wisconsin
Milwaukee, Wisconsin 53211
Address Correspondence To:
James B. Stiehl, MD
575 W River Woods Parkway, #204
Milwaukee, Wisconsin 53212
414-961-6789
Email: jbstiehl@aol.com
ANATOMICAL CONSIDERATIONS:
The patella is a sesamoid bone that functions to guide the forces of the quadriceps
muscles around the distal end of the femur.(Hehne, 1990) The patellofemoral articulation
is complex with shifting areas of contact throughout the range of motion. The femoral
condyles have a dual articulation with the patella and tibial condyles adding to their
complexity. Finally, the structures of the patellar tendon and extensor mechanism have a
balanced origin and insertion and torque generation to perfect this articulation. The
transmission of forces then depends on numerous factors, including the vectors of pull of
the various quadriceps muscles which hold the patella centered in the intercondylar
groove. For example, the vastus medialis has an origin more medially and dorsally which
tends to hold the patella flush against the femur even in extension and to balance the
lateral pull of the Q angle of the extensor mechanism. Tibial internal rotation with knee
flexion casues this Q angle to disappear.
The lateral retinaculum inserts onto the ventral surface of the lateral patella,
which if imbalanced has important pathological consequences. With flexion, the patella
sinks into the intercondylar groove enhancing the pull of the lateral retinaculum and
attatched vastus muscles. Any vastus medialis imbalance results in both lateral
displacement and increased lateral condylar contact pressure as the lateral condyle rises
to resist this displacement.
Another important mechanical phenonomen results when the knee flexes to 90.
At this point, the extensor mechanism tends to pull the distal femur dorsally on the
proximal tibia and is resisted by the collateral ligaments, joint capsule and the anterior
cruciate ligament. However, the vastus muscles change their vector of pull and become
knee flexors similar to the hamstring tendons. This force which is exerted through the
patella retinaculum causes the proximal tibia to translate dorsally as well and has been
characterized as the “tibial shear” force.
The points of insertion of the patella tendon and quadriceps tendon are at different
levels in the sagital plane of the patella.(Figure 1) For the quadriceps tendon, it attaches
to the patella closer to the joint surface than the patellar tendon. The levering effect of
the patellar tendon implies that the force transferred through this tendon is significantly
less than the quadriceps mechanism. The other issue is the transfer of force of the
quadriceps mechanism to the distal femur beginning at 70 of flexion. This tends to limit
any increase in retropatellar pressure after 70 of flexion as these forces are now taken up
by the quadriceps mechanism.
BIOMECHANICAL CONSIDERTIONS OF THE PATELLOFEMORAL JOINT:
Numerous biomechanical studies have analyzed the patellofemoral joint and have
suggested an important levering function as opposed to a simple pulley spacer.(Ahmed,
et.al., 1987; Andriacchi, et.al., 1986; Draganich, et.al., 1987; Hehne, 1990) Factors
related to patella kinematics are the comparison of the femorotibial and patellofemoral
contact and the angular orientation of the patella and the patella ligament.
With
increasing flexion, the effective moment arm decreases because the patella ligament
swings posterior as the patella falls into the median femoral groove, reducing the actual
moment arm. Also the patellofemoral contact migrates superior decreasing the
mechanical force advantage of the patella lever.
Patella tendon length is important as
increasing this length moves patellofemoral contact inferior causing a greater moment
arm and enhancing the levering of the quadriceps tendon. In result, there is a higher
patellar force concentration and a greater transmission to the patella ligament. At 82 of
flexion in normal knees, the quadriceps tendon wraps around the distal femur, which
rapidly reduces the amount of force transmitted through the patella and patella
tendon.(Yamaguchi,et.al.,1989)
The extensor moment generated across the patellofemoral joint is multifactorial
being related to tibiofemoral contact, the quadriceps/patellar tendon force ratio and the
patellar tendon angle. Draganich, et.al. have found that the largest extension moment of
the patellofemoral joint occurs at 15-30 of knee flexion.(Figure 2) This rapid increase in
tension results from the posterior movement of the location of the femoral-tibial contact
increasing the patellofemoral lever arm. Furthermore, the direction of pull of the patellar
tendon which is anterior at full extension, moves significantly posterior from 20-90 of
flexion. Again the posterior direction of pull on the patellar tendon results in the
posteriorly directed “tibial shear” force which resists any quadriceps-to-patellar tendon
force to pull the proximal tibia ventrally or anterior.(Ahmed, et.al., 1987; Draganich,
et.al., 1987; Huberti, et.al., 1984)
Komistek, et.al. have evaluated lower extremity joint reaction forces using Kane’s
method of dynamics.(Komitek, et.al., 1998) Figure 3 This approach uses known variables
such as ground reaction forces, gravitional forces, inertional forces of the lower extremity
and relative joint motions determined form gait lab and fluoroscopic measurements.
From a three dimensional model of the lower extremity, joint velocities and angular joint
velocities were used to solve differential equations which output the joint reaction forces
under various load bearing conditions. For the patellofemroal joint this measurement was
on the order of 0.2 to 0.4 times body weight during normal walking. It is believed this
calculation is representative as similar solutions for the hip joint in the same model
closely approximates results from telemetric measurements. Future efforts are needed to
assess other functional activites and those situations of prosthetic substitution.
PATELLOFEMORAL KINEMATICS IN TOTAL KNEE ARTHROPLASTY
The normal biomechanics of the knee joint can be significantly altered in total
knee arthroplasty by substantial changes to the normal anatomical
relationships.(Andriacchi, et.al., 1982, 1987) In general, the most substantial alterations
occur resulting from removal of one or both cruciate ligaments and from the prosthetic
shape of the femoral component that fails to recreate the normal spatial relationships of
the patellofemoral joint with the expected kinematic function. Removing the cruciate
ligaments causes a joint line elevation which has been shown to reduce flexion moments
and disrupt the normal femoral tibial contact or “femoral rollback” seen with knee
flexion. Both of these changes diminish the normal levering effect of the patellofemoral
joint. Specifically for the patellofemoral joint, moving the patellofemoral joint contact
superiorly with joint line elevation reduces the mechanical advantage of the patellar
lever. This cannot be improved by increasing the thickness of the patella, as the force
transfer of the extensor mechanism moves to the quadriceps tendon after 35 of flexion.
Thusly, at high degrees of flexion, patella thickness has minimal effect. .Finally, the
effect of prosthetic design on these various problems is very poorly understood. Suffice
it to say that no known total knee prosthesis to date has restored the normal extension
function of the patellofemoral joint.
In total knee arthroplasty, in vitro studies have evaluated sagital plane
patellofemoral contact area in normal and total knee arthroplasty. Huberti and Hayes
evaluated 12 human cadaver knees finding distal patellofemoral contact at 20 flexion
and proximal migration to the most proximal portion of the patella at 120
flexion.(Huberti, et.al., 1984) Takeuchi, et. al. studied patellofemoral contact in cadavers
with six total knee types finding an inconsistent superior or inferior shift of the contact
area with flexion.(Takeuchi, et.al., 1995)
Stiehl, et. al. used dynamic video fluoroscopy under weight bearing conditions to
investigate sagital plane patellofemoral kinematics in total knee arthroplasty.(Stiehl,
et.al., 1995) Patellar ligament rotation, which measures the angle formed by the patellar
tendon and the longitudinal axis of the tibia started at 16 extension in normal knees and
progressed to 0.
Prosthetic knees had a decreased angle in extension, which also
progressed to 0 with flexion. Patella axis rotation, which compared the angle between
the patellar tendon and the sagital axis of the patella, increased with knee flexion in both
normal and total knees, but was greater than normal in total knees in full flexion. Two
abnormalities first identified in that study included abnormal patellar separation in full
extension and a pie-shaped or wedge gap opening at the distal pole of the patella as the
patellar prosthesis articulated on the more superior surface of the dome shaped patellar
prothesis.
Stiehl, et. al. previously investigated the LCS mobile bearing anatomical patella
comparing the results with or without posterior cruciate ligament sacrifice.(Stiehl, et.al., ,
1997, 2000) Patellofemoral contacts of both mobile bearing implants were similar to
normal but tended to be more inferior with higher degrees of flexion which contrasted
with the superior position seen on dome shaped implants. Patella ligament rotation was
lower than normal in the mobile bearing implants reflecting the posterior femorotibial
contact in extension and anterior translation beyond 60 flexion. Patellar axis rotation
angles were similar for normal and total knees.
Stiehl, et.al. used invivo video fluoroscopy to investigate patellofemoral
kinematics of multiple possibilities comparing the position of patellofemoral contact,
patellar tilt angle which measured the change of the axis of the patella in relation to the
long axis of the tibia, and separation of the patellofemoral joint in extension.(Stiehl, et.al,
2001) Figure 4,5 This was compared with normal, anterior cruciate deficient knee, fixed
bearing posterior cruciate retaining total knees with a dome patella, fixed bearing
posterior stabilized total knees with a dome, and the mobile bearing rotating platform
LCS, with and without patellar resurfacing. In that study, patellofemoral contact position
moved superior in normal and ACLD knees in virtually identical fashion. All TKA
experienced a more superior contact position at full extension through 30o of knee
flexion, and this pattern continued for subjects having a fixed bearing PCR or PS TKA at
60 and 90o of knee flexion. However, the mobile bearing resurfaced and unresurfaced
patellae with the posterior cruciate sacrificing LCS implant had contact positions similar
to normal knees at 60 and 90 flexion. For the entire group the most superior overall
patellofemoral contact pattern was determined for subjects having a posterior cruciate
retaining total knee. The authors attributed this finding to a variety of issues including
the loss of normal roll-back with many posterior cruciate retaining total knees, subtle
joint line elevation, and certain anatomical issues that may be related to surgical
technique.
The patellar tilt angles of the domed patellae were similar with posterior cruciate
retention or substitution and were greater than normal or ACLD knees. Again, this may
reflect joint line elevation with posterior cruciate sacrifice or abnormal posterior
femorotibial contact positioning seen with posterior cruciate retaining knees. The patellar
tilt angles of unresurfaced mobile bearing TKA were most similar to normal knees.
Subjects having a resurfaced MB TKA experienced larger patellar tilt angles compared
with subjects having a MB unresurfaced TKA, but were lower than those values
determined for fixed bearing TKA having dome-shaped patellae. This may reflect the
relative anatomical shape of the mobile bearing patella. The authors concluded that
patellar tilt angles produced trends comparable to patellofemoral contact analysis with
higher patellar tilt angles correlating with more superior patellofemoral contact. They did
not investigate joint line elevation or possible changes of patellar tendon length in their
study.
Patellofemoral separation in extension was seen in several kinematic studies and
may be explained in part by femorotibial contact, which tends to be more posterior for
total knees and some ACLD knees.(Komistek, et.al., 1998, 2000; Stiehl, et.al. 2001) The
highest incidence and magnitude of separation was seen in total knees with posterior
cruciate retention, which may reflect absence of the anterior cruciate ligament and
posterior femorotibial contact in extension. Posterior cruciate substituting total knees
also demonstrated a substantial number with this finding but only half the number seen
with posterior cruciate retention. Dennis, et. al. has shown that femorotibial contact with
PS TKA will, on average, be more anterior than PCR TKA, which could explain this
difference.(Dennis, et.al., 1996) Interestingly, in Stiehl’s study, none of the unresurfaced
or resurfaced anatomical MB TKA demonstrated separation, and were comparable to
normal knees. The clinical implication of patellofemoral separation is unknown but
could explain certain “clunks” that some patients experience.
Miller, et.al. used a cadaver method to recreate patellofemoral forces with various
prosthetic implant techniques.(Miller, et.al. 2001) They found that with a medial
unicompartment arthroplasty with a meniscal bearing prosthesis where both cruciate
ligaments were preserved, the patellofemoral forces were not different from the normal
knee. With total knee arthroplasty and posterior cruciate retention, the patellar forces
were significantly lower than the intact knee at 20 and 40 of flexion and significantly
higher at 100 and 120 of flexion. For the posterior stabilized arthroplasty with incision
of both cruciate ligaments, patellar forces were diminished at 20 of flexion but were
comparable to normal at higher degrees of flexion. They assessed the patellar tendon
angle which measures the angle of the shaft of the tibia with the patellar tendon and
found changes which could impart could explain alterations of the patellar levering
mechanism. The unicondylar arthroplasties remained comparable to normal knees
throughout range of motion but in the posterior cruciate retaining arthroplasties, in 20
and 40 of flexion, the angles were less while in 100 and 120 of flexion, they greater
than normal. This could be explained by the well recognized sagital plane kinematic
abnormalities of the posterior cruciate retaining total knees which are posterior in
extension and move anteriorly in deep flexion. For the posterior stabilized implants, the
patellar tendon angle was less than normal at 20, 40, and 60 of flexion, but normal at
higher degrees of flexion. Again, this may reflect a posterior femoral tibial contact in near
extension.
DISCUSSION:
From this brief review of the literature, much is known about the function of the
patellofemoral joint. However, controversy still persists about various orthopaedic
treatments of anterior knee pain, which is a common office complaint. Two of the most
common operations are lateral retinacular release of the patella and anteromedialization
of the proximal tibia. In the field of reconstructive surgery, total knee arthroplasty is one
of the most commonly applied surgical methods. Yet, the existing surgical techniques in
each of these areas probably do not restore the function of the knee to normal as recorded
by gait analysis or other functional criteria. The parameters of rehabilitation also are
poorly understood. For example, how much muscle performance of the vastus medialis
is needed to correctly balance the patellofemoral articulation and can this function be
measured and assessed over a period of time.
The biomechanical analysis of the patellofemoral joint will require more
sophisticated investigations in the future. Invitro cadaveric models are helpful at least
with parametric studies to demonstrate obvious trends. However, we must understand the
forces of articular surface loading and how these become altered in disease states and
with mechanical alterations from trauma, abnormal development, or prosthetic
replacement. In the future, computerized modeling will be needed to generate solutions
to these complex problems. Only then can we plan appropriate surgical procedures that
will guarantee effective outcome in most cases.
BIBLIOGRAPHY
Ahmed, A.M., Burke, D.L., Hyder, A., 1987. Force analysis of the patellar mechanism.
Jl. Orthop Res 5, 69-85.
Andriacchi TP, 1993. Functional analysis of the pre and post-knee surgery: total knee
arthroplasty and ACL reconstruction. J. Biomechanical Engineering 115, 575-581.
Andriacchi , T.P., Galante, J.O., Fermier, R.W., 1982. The influence of total knee
replacement design during walking and stairclimbing. J Bone and Joint Surg. 64A,
1328-.
Andriacchi TP, Stanwyck S, Galante JO, 1996. Knee biomechancis and total knee
replacement. J. Arthroplasty 1, 211-219.
Dennis DA, Komistek RD, Hoff WA, Gabriel SM, 1996. In vivo kinematics derived
using an inverse perspective technique. Clin Orthop 331, 107-117.
Draganich LF, Andriacchi TP, Andersson GBJ, 1987. Interaction between intrinsic knee
mechanisms and the knee extensor mechanism. J. Orthop. Res. 5, 539-547.
Hehne H-J, 1990. Biomechanics of the patellofemoral joint and its clinical relevance.
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Huberti HH, Hayes WC, 1984. Patellofemoral contact pressures. The influence of Qangle and tendo-femoral contact. J. Bone and Joint Surg. 66A, 715-724.
Huberti, H.H., Hayes, W.C., Stone, J.L., Shybut, G.T., 1984. Force ratios in the
quadriceps tendon and ligamentum patellae. J Orthop Res 2, 49-54.
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patellofemoral contact positions. J. Clinical Biomechanics 15,29-36 .
Komistek RD, Dennis DE, Mabe A, 1998. Invivo determination of patellofemoral
separation and linear impulse forces. Der Orthopaede 27: 612..
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LEGEND
Figure 1. Diagram demonstrates the different forces of the quadriceps: F 1; and patellar
tendon: F2; when the insertion of the quadriceps and origin of the patellar tendon are at
different levels on the patella causing the R, the resulting pressing force which is the sum
of F1 and F2; a1 is the lever arm of F1, a1 + b = lever arm of F2 (From Hehne, 1990)
Figure 2. For a given quadriceps force, the extension torque peaks at 25 of flexion
based on the femorotibial contact position, quadriceps-to-patella force transfer, the
changing angle of the patellar ligament. (From Draganich, et.al.1987)
Figure 3.
Joint reaction forces of the hip, knee, ankle, and patella during normal
gait(From Komistek, et.al., 1998)
Figure 4. Patellar contact position from the patellar sagital plane midline from extension
to 90 flexion. (From Stiehl, et.al. 2001)
Figure 5.
Patellar Tilt Angles from extension to 90 flexion with various
conditions.(From Stiehl, et.al. 2001)