PATELLAR INSTABILITY IN TOTAL KNEE
ARTHROPLASTY
James B. Stiehl, MD
Midwest Orthopaedic Biomechanical Laboratory
Columbia St.Mary’s Hospital
Milwaukee, Wisconsin
Address Correspondence To:
James B Stiehl, MD
575 W River Woods Parkway, #204
Milwaukee, Wisconsin 53212
414-961-5678
Fax: 414-961-6788
Email: jbstiehl@aol.com
Introduction:
Instability of the patella may occur after total knee arthroplasty and has been
identified either with or without prosthetic resurfacing of the patella. Subluxation is more
common than dislocation, but the incidence of symptomatic instability leading to
reoperation is low ranging from 0.5% to 0.8% in several referral centers.22,25 In a recent
multicenter study of LCS mobile bearing total knee arthroplasty, only 6 (2.3%) of 259
revisions related to patellar instability and of the overall group this accounted for a
revision rate of 0.1% at average of 5.7 years followup.27 The etiology of this problem is
multifactorial but can be most commonly related to a host of problems including
component malposition, soft tissue imbalance, trauma, and excessive valgus alignment of
the knee. In addition, problems related to surgical technique such as tibial or femoral
internal rotation, increasing patella-patellar implant composite thickness, and increasing
femoral component dimension can cause patellar instability. This review will explore
each of these issues and describe methods of treatment.
Etiology:
Component Malposition:
One of the most common causes of patellar instability is component malposition
at the time of surgical implantation.1,2,12,15,17 Any tendency to place the femoral or tibial
components in abnormal internal rotation in the transverse plane can predispose to
patellar subluxation essentially by increasing the Q angle at the knee joint.(Fig 1) Much
has been written about correct positioning of these components, either to increase the
understanding of the normal anatomical relationships or to advance methods that enable
the surgeon to place the components correctly.
Surgeons have performed the distal femoral resection based on anatomical
landmarks which guide the amount of normal femoral external rotation needed to match
the normal proximal tibial slope of about 93 degrees in the knee joint and maintain the
normal femoral position.5 Originally, the posterior condylar axis was the resection plane
for making the posterior condylar cuts but this tended to place the femoral component in
internal rotation leaving a trapezoidal flexion gap particularly if the proximal tibial cut
was perpendicular to the long axis of the tibia. Surgeons then attempted to external rotate
the posterior condylar cutting jig a fixed amount of 3 to 5 to give the appropriate
femoral external rotation and a balanced symmetrical flexion gap. Berger, et.al. found
the relation of the posterior condylar axis to the surgical transepicondylar axis (point of
lateral epicondyle to the sulcus of the medial epicondyle) to average 3.5º for males and
0.3º for females which was a highly significant statistical difference. However, the
clinical angle using the prominence of the medial epicondyle was 4.7º for males and 5.2º
for females. Significantly, the variance could range from 1º to 9.3º.4 Mantas, et.al. found
in normal femurs that the range of posterior condylar axis reference to the
transepicondylar axis ranged from 0.1º to 9.7º.19
A more reliable method has been to use the transepicondylar axis which has been
shown to be more consistent compared to the posterior condylar axis. Stiehl, et.al. has
shown that the transepicondylar axis has a perpendicular relationship anatomically to the
long axis of the tibial shaft.27 (Fig 2) As the transepicondylar axis closely approximates
and parallels the axis of knee flexion, Insall, et.al. advocated cutting the posterior
condyles parallel to this axis will place the femoral intercondylar groove in the normal
anatomical position.15,25 Similiarly, the Whiteside line drawn from the femoral
intercondylar groove down to the center of the femoral notch is virtually perpendicular to
the transepicondylar axis and also represents a suitable technique.
Fehring compared
the Insall method with the measured resection method of distal femur resection using a
fixed posterior condylar reference guide finding that the measured resection technique
resulted in rotational errors of at least 3º in 45% of knees.9
Similiarily, Olcott and Scott
found that the transepicondylar reference most readily determined a balanced flexion
space while using 3º rotation off the posterior condyles was least consistent.21
A more subtle problem with femoral component placement is the location of the
anatomical femoral groove of the patient compared to that of the prosthetic component.
Eckhoff, et.al. have shown that this groove in the anatomical specimen tends to be 2.5
millimeters lateral on average to the anatomical midplane of the distal femur based on the
dimensions of the condyles and position of the femoral notch but could be up to 8
millimeters in outliers.7(Fig 3) Most contemporary implants have a symmetrical design,
such that condyles are roughly equal in dimension, and the placement of the prosthetic
femoral groove bisects these condyles. By placing the femoral prosthesis in what may
appear to be the anatomically correct or symmetrical position in relation to the femoral
notch, the surgeon may actually be medializing the femoral intercondylar groove. A
technical solution to this problem it to mark the patient’s femoral intercondylar groove
during preparation and then attempting to match this position with that of the prosthesis.
(Fig 4) This may cause some overhang of the lateral femoral condyle, but that would be
preferable to abnormally increasing the Q angle.
Tibial component placement can also be problematic, especially if the surgeon
attempts to cover the tibial on the medial side. This can lead to placing the tibial
component into relative internal rotation to the correct axis of the patient’s knee leading
to lateral placement of the tibial tubercle and aggravation of the Q angle. Eckhoff, et.al.
have shown variability of this knee axis and there is a subgroup of osteoarthritic patients
where there is actually external version of the tibial in relation to the femur.8 This
problem tends to magnify the amount of varus deformity that may accrue with disease.
Several authors favor centering the tibial component on the medial third of the proximal
tibial tubercle and this usually works. Another method is to draw a line from the
patient’s femoral intercondylar groove down to the proximal tibia and matching that
position throughout the procedure. That can be particularly useful with methods that use
a sloped cut of the distal tibia, as making the cut out of plane may aggravate alignment
errors. The best way however, is to carefully trial the implants on insertion make certain
that the tibial articulation is perfectly midline and not rotated onto the anterior surface of
the medial tibial insert.
Patellar component malposition usually reflects technical error in cutting the
patella. The effort is to make a symmetrical hockey-puck shaped structure that has equal
thickness top to bottom and side to side.(Fig 5) Under resection of the lateral facet or the
distal pole will lead to tightness of the lateral retinaculum and a tendency to sublux
laterally.13,23 Grace and Rand were clearly able to show increasing patellar thickness or
stuffing the joint could lead to patellar lateral instability.11 Another problem is centering
the patellar dome on the midpoint of the patellar remnant and not shifting the dome to the
most medial edge.18 In the normal patella which averages 37 millimeters in width, the
average medial facet is 14 millimeters while the average lateral facet is about 23
millimeters. Lateral dome placement increases the Q angle and also aggravates lateral
retinacular tension. Another option to solve this issue is to use an eccentrically shaped
dome or an anatomical mobile bearing device such as the LCS which appropriately
matches patellar anatomy.(Fig 6) Finally, a poorly studied issue of the patellar
articulation is the shape of the prosthetic intercondylar groove in relation to the
anatomical distal femur. No doubt, earlier designs were more “boxy” in shape and
tended to add metal thickness to the intercondylar groove, in effect stuffing the joint.
Recent total knee designs have ameliorated this problem by designing deeper prosthetic
intercondylar grooves, which will likely be demonstrated in dramatic reduction of patellar
complications in long term studies. The LCS mobile bearing prosthesis had this concept
as an initial design goal and has realized a patellar component survivorship of 98.5% at
14 years followup.26(Fig 7)
Limb Malalignment:
Several authors have noted the potential of a valgus knee to predispose to patellar
instability. Not uncommonly, these knees have a chronically subluxed or dislocated
patella. Often there is lateral retinacular tightness associated with the long standing
deformity, and if not addressed with release can cause subluxation. The lateral condyle
in a valgus deformity may be smaller than normal in dimension. If the posterior condylar
axis is used for distal femoral resection, there may be a tendency to internally rotate the
femoral component, and increase the Q angle as noted above. With the use of
appropriate instrumentation, especially intramedulary femoral guides, the chance of
creating a postoperative valgus deformity is diminished and this problem may be even
further reduced with the advent of computer assisted navigation of alignment that has
shown to be even more precise.
Soft Tissue Imbalance:
Lateral retinacular tightness remains a subtle cause of patellar instability, but
usually does not result in a clinical problem by itself. A number of authors have clearly
shown that surgical technique such as basing the femoral resection on the posterior
condylar axis creates femoral internal rotation which will lead to the need for lateral
release. Therefore, if a patient has patellar instability, the surgeon must look for other
causes or problems than simply tightness of the lateral retinaculum. A chronically
dislocated or subluxed patella is the exception, however, and retinacular tightness can be
the primary source in this case. Unfortunately, these cases may be complicated also by
medial retinacular weakness or atrophy, and every attention to the details of
reconstruction are needed. In these unusual cases, attention to every detail is needed such
as using a lateral parapatellar approach, carefully aligning the implants, and then
positioning the implants to optimize patellar tracking with the final need to reef or
reconstruct the stretched medial soft tissues.
Other Causes:
Medial retinacular weakness or disruption is possible after total knee arthroplasty
and may result from an expanding hematoma, inadequate surgical closure, overintensive
physical therapy, or injury.14,15 Surgical repair will be needed if medial laxity is
documented with patellar subluxation. Rare causes of patellar instability have resulted
from surgical misadventure, such as placing the right femoral component in the left knee,
and placing a ridged anatomical patella component such that the ridge was parallel to the
transverse joint plane and not verticle.6,10
Clinical Findings:
The hallmark of patellar instability is anterior knee pain aggravated stressful
activites such as stair climbing or rising from a chair. With instability, there can be
dramatic giving way or buckling of the knee while subluxation will cause the sensation of
the knee slipping out of place. Palpation of the extensor mechanism throughout passive
and active range of motion will reveal defects in continuity. Also, the patient may be able
to identify areas of localized tenderness. Dislocation or subluxation may be detected by
palpating the patella through the range of motion. Provocative maneuvers such as
attempting to sublux the patella laterally during active flexion can also elicit pain or
apprehension. Also patella alta should be observed as this may make the patella
somewhat higher in the femoral groove. Finally, there are those cases who have
asymptomatic lateral subluxation without any objective findings except vague medial
knee pain. This problem should be identified with radiographic views as many patellar
implants are subject to accelerated polyethylene wear with this abnormal articulation.
Radiographic Findings:
Radiographic evaluation of the patella relies primarily on the lateral view and the
sunrise or Merchant’s view of the patella. The symmetry of the patellar cut and thickness
of the patella composite is apparent and may be compared with the opposite normal
patella. The infrapatellar view will demonstrate the position of the patellar component in
relation to the trochlear sulcus, either centralized, tilted, subluxed, or dislocated. Tilt can
be defined as medial or lateral depending on the relation to the femoral condyles.
Subluxation can be measured as displacement from the center of the prosthetic femoral
intercondylar groove. Component rotation can be determined with computed
tomography scans through the knee joint. Berger, et.al. have identified four scans needed
for this determination which are scans through the medial and lateral epicondyles, the
tibial plateau immediately below the tibial base plate, the tibial tubercle, and through the
tibial insert.3 The femoral component rotation is determined by measuring the angle
formed by a line drawn through the medial and lateral epicondyles and the line
connecting the posterior flanges of the implant. Tibial component rotation is determined
by first finding the geometric center of the proximal tibia and then superimposing this
point onto the image with the tibial tubercle. A line is drawn to the highest point of the
tubercle and this becomes the tibial tubercle axis. This axis is then placed on the image
with the tibial insert. A line is drawn perpendicular to the posterior surface of the tibia
insert and when compared with the tibial tubercle axis, becomes the amount of tibial
rotation. According to this method, the normal amount of tibial internal rotation is 18.
Treatment:
Prior to any surgical intervention, conservative methods should be tried including
quadriceps exercises, bracing, and avoiding activities that may aggravate instability
symptoms. With time, scarring of the retinacular tissues can often lead to resolution of
symptoms. With chronic instability symptoms or frank dislocation, surgical intervention
is necessary. Careful assessment of all possible prosthetic causes is done such as
component malrotation, limb malalignment or soft tissue problems about the patella.
Component malposition will require revision. Soft tissue problems may require proximal
realignment, which is done with a lateral retinacular release and medial vastus
advancement.14,24,25 Distal bone realignment of the tibial tubercle may also be done in
combination with the proximal realignment. Numerous methods have been described for
the distal realignment including a modified Trillat procedure or using a fairly long
osteotomy as described by Whiteside.28 Great care must be taken to insure that an
adequate piece of bone (at least 8 centimeters) is taken and that apposition and fixation is
optimal. Wound complications, rupture of the patellar tendon, and fracture of the bony
remnant are inherent risks with this approach, and must not be undertaken lightly.
Results:
Insall described a method of long lateral release from inside the joint outwards,
and then imbricated the medial vastus retinaculum over at least 50 to 75% of the width of
the quadriceps tendon. (Fig 8) Merkow, et..al. reported no recurrences using this
technique in 12 cases, but noted one case of skin necrosis and one patella fracture after
this approach.20 Grace and Rand reported the results of 25 knees with 14 having a
proximal realignment of which 4 recurred.11 Of nine knees treated with both proximal
and distal realignment, no recurrent dislocations were noted but two sustained distal
patellar tendon ruptures. Two other cases had revision of components, and one of these
had further subluxation. Kirk, et.al. used a modified Trillat procedure in 15 cases of
patellar dislocation and noted no recurrences or problems with the patellar ligament.16
Case Report #1
A 54 year old morbidly obese female underwent total knee arthroplasty using a
mobile bearing total knee arthroplasty. Component position and limb alignment were
deemed optimal during the procedure and the patella component tracked well with the
“no thumbs” technique. Nonetheless, postoperatively, she developed symptomatic
subluxation with buckling episodes and chronic anterior knee pain. This was confirmed
with a Merchant’s view of the patella.(Fig 9) At reoperation, the patella again was noted
to track normally on passive motion. A proximal realignment was done with lateral
release and medial imbrication. The knee was then held in a knee immobilizer for six
weeks followed by rehabilitation. Within four months, symptoms had recurred to the
original level. At this point, a combined proximal and distal realignment was done. The
distal alignment was a long tibial crest fragment measuring 1.5 cm by 8 cm which was
fixed with screws.(Fig 10) The leg was immobilized again for six weeks followed by
rehabilitation. After four years followup, there has been no recurrence of symptoms.
Case Report #2
A 69 year old female was underwent revision of a failed unicondylar arthroplasty
done for osteoarthritis. Following the revision, the tibial base plate was noted to be in 9
of varus and there was frank dislocation of the patella with chronic instability
symptoms.(Fig 11) A proximal realignment was attempted with early recurrence of
dislocation. A computed tomograph was done of the knee revealing a femoral
component internal rotation of approximately 9.(Fig 1) This compared to the normal
rotation of the TEA to the posterior condylar axis of 3.(Fig 12) The tomogram through
the knee prosthesis revealed the patella to be perched on the lateral femoral condlyle.
Revision arthroplasty was needed finding marked internal femoral rotation, apparently
done to match the severe varus placement of the tibial tray. Another finding that is
typical of this error is that internally rotating the femoral component this dramatically left
the flexion space on the lateral side lax by 12 to 14 millimeters, functionally exaggerating
the genu valgum.(Fig 13) All these problems could be resolved with satisfactory
placement of revision implants, though the knee was immobilized in extensions for three
weeks to allow for healing of the Insall medial retinacular realignment.(Fig 14)
Discussion:
Patellar instability is an uncommon sequelae from total knee arthroplasty.
Surgical technique is probably the most important cause of this problem with abnormal
component rotation or limb malalignment being the common errors. With evolutionary
methods of technique and prosthetic design, instability now has become a rare incidental
finding after total knee arthroplasty. Surgeons faced with this problem must make every
effort to define the exact cause. Component malposition must be treated with component
revision, while soft tissue imbalance can be managed with proximal soft tissue
realignment. Distal tibial tubercle transfer may be utilized but must be done with great
care to avoid complications of skin necrosis or patellar ligament disruption.
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LEGEND
Fig 1. CT scan through transepicondylar axis reveals femoral component internal
rotation of 9 with subluxation of the patella in a patient with patellar instability.
Fig 2. Anatomical specimen with pin through transepicondylar axis demonstrated the
perpendicular relationship of the TEA to the long axis of the tibial shaft.
Fig 3. Position of the femoral sulcus in relationship to the transverse midplane of the the
knee where the center bisects the posterior femoral condyles.
Fig 4. Marking the femoral sulcus as a landmark for centering the femoral component,
and as a rotational reference in extension for a longitudinal matching mark on the
proximal tibia.
Fig. 5 Careful assessment of the thickness and dimension of the patella to make certain
that face removal creates a “hockey puck” geometry.
Fig 6. Eccentric polyethylene dimension matches natural geometry of the patella to
facilitate patellar tracking.
Fig 7. Modern femoral component design optimizes the position of the intercondylar
groove attempting to match the anatomical position with prosthetic replacement.
Fig 8. a.) Insall proximal medial soft tissue realignment using medial parapatellar
incision; b.) vastus medialis and retinaculum are reefed and advanced 50-75% over the
central patellar tendon.
Fig 9. Sunrise view demonstrates patellar subluxation of LCS mobile bearing implant.
Fig 10. Tubercle osteotomy of proximal tibial with medial translation of long tubercle
fragment of approximately 8 millimeters and fixed with screws.
Fig 11. Anterior posterior view of knee after revision of unicondylar arthroplasty
resulting in severe varus position of the tibial base plate and associated femoral internal
rotation which caused chronic patellar instability.
Fig 12. CT Scan of normal knee reveals 3 difference from the TEA to the posterior
condylar axis.
Fig 13. Intraoperative image demonstrates tibial varus and chronic instability of the
flexion space on the lateral side.
Fig 14. a.) Operative view of the Insall proximal soft tissue realignment; b.) AP
radiograph of postoperative revision for component malposition.