Abstract of the Ph.D. thesis
THE CHANGES OF KNEE BIOMECHANICS
FOLLOWING MEDIAL
UNICOMPARTMENTAL KNEE
ARTHROPLASTY
By Tibor Gunther M.D.
Doctoral School of Semmelweis University
Semmelweis University
Faculty of Physical Education and Sport Sciences
Pedagogical (Sport Science) Doctoral School
Supervisor: Dr. Anikó Barabás
Associate Professor
Budapest
2001.
1. INTRODUCTION
The knee joint is our body's largest and most complicated
articulation, which does a gliding-rolling movement through flexion,
accompanied by rotation. The bony parts supply the freedom of this
movement, from the gliding-rolling point of view. The complexity of it is
organised by the ligaments, tendons and muscles, that is the soft tissues.
However from anterior-posterior view the joint's biomechanical axes and
angle are determined by the bony parts, the shape of the condyles and the
deviation angle between the diaphysis and the joint line.
Number of studies can be read about the complexity of the knee joint's
movement, mainly under physiological circumstances. Far fewer works
have been published about the biomechanical analyses of pathological,
arthritic or prosthetised knees. Since in our country comparing with the
international data, unicondylar knee arthroplasties have been implanted with
a greater proportion to total knee prostheses and because this prosthesis type
presumably has the less influence on the biomechanics of the knee joint,
raises the need to prove this with biomechanical measurements.
Goodfellow has published the complex gliding-rolling movement
of the knee joint with the help of the so called contact or identical points
(figure 1.).
Figure 1.: The identical points of the femur and the tibia as they touch each
other at certain angles of flexion.
Under the same rotational position always the same femur and tibia points
contact each other in certain angles of flexion. These are called the
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identical or contact points. If we mark these points at certain angles of the
flexion before the prosthesis implantation, we can compare and measure the
identical points resulted following the procedure. By my opinion in this
way the path of the gliding-rolling movement can be analysed, including the
possible changes of these lengths, and the biomechanics of the knee joint,
from the gliding-rolling movement's point of view.
The mechanical axis of the lower extremity is a line drawn form
the centre of the femoral head to the centre between the medial and lateral
malleolus, which under physiological circumstances goes through the centre
of the knee joint. The anatomical axis of the femur deviates six degrees on
the average from the lower extremity's mechanical axis.
In 1980 Johnson published, how the deviation of the mechanical axis
influences the load distribution of medial and lateral compartments. Under
physiological circumstances, that is six degrees valgus of the femur's
anatomical axis, the 60% of the load goes through the medial and only 40%
through the lateral compartment. Six degrees of deviation in varus direction
causes 80% load on the medial compartment, on the other hand 6 degrees of
valgus deformation 80% of the lower extremities load goes through lateral
condyles (figure 2.).
Figure 2.: The connection between the mechanical axis and the load
distribution according to Johnson
2. THE AIM OF THE STUDY
The aim of my thesis was to analyse the biomechanics of the knee
joint, more precisely to study the effect of the unicompartmental knee
arthroplasty on the one hand from the gliding-rolling point of view and on
the other hand from the functional results of the correction of the deviation
angles.
1) The gliding-rolling movement had been measured in the first part of my
thesis in two ways, one was done on the cadaver knees, the other was
performed on side views of X-ray films made pre- and postoperatively.
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2) The greater number of unicompartmental knee prosthesis operated in
Hungary, the newer operating possibilities and the uncertainty of the
unicondylar knees' success rate raise the question if the correction of the
deviation angles in the knee influences the postoperative functional
outcome.
Thus in the second part of my thesis, the aim of my work was to measure,
how valuable, essential and possible to make an effort to correct the
deformation's deviation angle in unicompartmental knee prosthesis in order
to make the patient's movement, functional result the best following the
procedure.
3. MATERIALS AND METHODS
The gliding-rolling movement of the knee joint was measured by
two methods before and following the unicondylar knee prosthesis
implantation.
In the first group, measurements have been done on cadaver knees.
On 12 deceased's 23 knees, the contact points on the femur and the tibia
have been compared in 0, 45 and 90 degrees of flexion, before and
following medial unicompartmental knee arthroplasty. I have done the
procedure through the exposure, which we use in normal operations in our
department. The prosthesis was the Protetim unicondylar knee prosthesis
type, which we use, everyday in our practice. Following the exposure, with
the help of coloured headed pins, we marked the identical points both on the
femur and the tibia in 0, 45 and 90 degrees of flexion. Following this, we
carried out the implantation of the medial unicompartmental knee prosthesis
and measured the differences of the contact points in millimeters. The
movement of the points has always been compared to the preoperative site
(marked with the pins). Consequently the measured movements are never
changes compared to 0, 45 and 90 degrees of flexion, but always a distance
measured form the preoperative site. This way the gliding direction is
posterior pre- and postoperatively as well. The flexion angles were
measured and controlled with angle meter and we assured about the same
rotation position, through the whole movement. The deficiency effect of
the ligament cruciatum anterior - following the measurements of the
movement changes after the implantation of the prosthesis - was modelled
with cutting it out in full thickness and remeasure the movement changes in
0, 45 and 90 degree of flexion.
In the other group I measured the differences between pre- and
postoperative movements on side positioned X-ray films, done in standard
circumstances. Standard circumstances mean the same X-ray room, the
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same X-ray table and the same tube-film distance. The positioning of the
patients has been done by the same person, and following the development
of the films I controlled the proper projection of the two condyles of the
femur and also the tibia - fibula distances. If these two projections were not
proper we redid the X-ray films and in an irreproducible situation I dropped
the patient from the trial. In my opinion this way the standard
circumstances have been controlled by eight knees repeated X-ray and
remeasuring of films ("interobserver error").
I have done the measurements on these films correcting the distances
according to the magnification of the X-ray. Due to the prosthesis' metal
projection and the changing radius on the femur I could measure on the
tibial side only.
In the second part of the study the basis of the trial was the
patients, who have had a unicompartmental knee prosthesis implantation, in
our department in 1999. This way I analysed all the colleagues' patients,
however the indication criteria and the operating technique were the same.
I included all the medial and all the lateral unicompartmental arthroplasties.
The prosthesis type was the Protetim unicompartmental knee implant with
flat polyethilene tibial plateau.
For the objective analysis of the functional results, I have reviewed the
patients and recorded the clinical data with the help of the New Jersey Knee
Score. We have had the postoperative weight bearing AP knee X-rays also
at the same time. Regarding the X-rays we have to emphasize that both the
preoperative and the postoperative X-rays were weight bearing, using long
films, which extended from the mid part of the diaphysis of the femur to the
mid part of the diaphysis of the tibia. As a result of the special attention of
the radiological assistants, all of these X-rays were taken with parallel
forward looking feet, to avoid the influence of the rotation and the
retroversion of the tibia on the estimation of the varus-valgus deviation.
The anatomical and mechanical axes of the lower extremity are well known.
Given the length or shortness of films available in our institute and also
considering the maximum X-ray exposure of the patients, I have had the
option to measure properly the anatomical axes (the deviation between the
joint line and the long axes of the tibia and the femur). For this purpose I
considered the following values as normal 6 degrees of valgus angle on the
femoral side the and the most commonly accepted 3 degrees of varus on the
tibia.
In medial and lateral unicompartmental knee arthroplasties I
separately analysed the pre-, postoperative and corrected angles and
compared them with the New Jersey summarised knee scores, and also with
the results of the subgroups. The above results were analysed with the help
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of the Microsoft Excel programme and controlled with 1 & 2 sample t-test,
where the significance level was given as p<0.05.
4. RESULTS
In the cadaver study I examined on 12 deceased's 23 knees. The
average age of the people was 72 years, 5 of them were women and 7 men.
We managed to achieve acceptable results both on the femur and the tibia
(table 1.).
The average backward movement on the femur (as the result of 23
measurements) was 7.3 mm in extension, in 45 degrees of flexion it was
unchanged and in 90 degrees of flexion -0.13 mm, where the negative sign
means forward slip (and that's why this was marked negative in the table).
On the tibia the average backward movement following medial
unicompartmental knee replacement compared to the preoperative position
(also as the result of 23 measurements) was 3.91 mm backward movement
in extension, in 45 degrees of flexion it was unchanged and in 90 degrees of
flexion -0.65 mm backward slip. In this last measurement as on the femur,
we registered forward movement, that's why we marked it with negative
sign.
femur
tibia
Average backward Average backward Average backward
movement
in movement in 450 movement in 900 of
extension
of flexion
flexion
7.3 mm
0 mm
-0.13 mm
3.91 mm
0 mm
-0.65 mm
Table 1.: On cadaver knees the average backward movement on the femur
and the tibia in millimetres following medial unicompartmental knee
replacement compared to preoperative positions.
According to the above measurements the movement path on the tibia has
shortened with 4.6 millimetres.
Following the cutting through of the anterior cruciate ligament (ACL) I
measured further backward slip on the tibia, 1.7 mm in extension, in 45 and
90 degrees of flexion each was 1 millimetre.
In the X-ray study I measured 24 patients' knees, their average age
at the time of the surgery was 69 years, the sex ratio was 19 women and 5
men. For technical reasons, on the X-ray films (overprojection and the
radius changes of the contacting superficies, etc), I could make
measurements only on the tibia. Accounting for 20% magnification of the
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X-rays the backward movement in extension was 4.43 mm, in 45 degrees of
flexion it was 0.77 mm of forward movement and in 90 degrees of flexion
on the average 0.97 mm forward slip was detected. According to the
measurements on the X-ray films, the movement path has been shorthened
on the tibia following medial unicompartmental knee replacement on the
average from 9.43 mm (11.79*0.8) to 4.03 mm (5.04*0.8), which means on
the average 5.4 mm shortening.
The movement path's shortening did not mean symmetrical (equal
shortening from the front and from the back) changes. Both in the cadaver
knees and also on the X-ray measurements the backward movement was
greater in extension (in cadavers 3.91 mm, on X-ray films 4.43 mm) than in
90 degrees of flexion, while the forward slip was shorter (in cadaver knees
0.65 mm and on X-ray films 0.97 mm). According to the above
measurements, the movement path's backward movement was greater in
extension than the forward movement in 90 degrees of flexion. As the
change in 45 degree of flexion was minimal, thus all in all the movement
path primarily shortened, was placed backward, between 0-45 degrees of
flexion in both of the measurements. The above measurments have also
been proved by 1 sample t-test, as the points before the implantation were
different compared to the points postoperatively (p<0.001). On the other
hand, according to the statistical analyses the contact points did not change
significantly following medial unicompartmental knee replacement in 45
and 90 degrees of flexion (p>0.4 és p>0.3).
The standard circumstances have been controlled by eight knees repeated
X-ray and separately remeasuring of films by another colleague
("interobserver error"). In neither of the control groups were found
significant difference with 2 sample t-test.
The above movement changes, with addition of cutting through of the ACL,
make it obvious that the already shortened and backward placed movement
path is further shortened and placed backwards.
In the second part of the study, while examining the correction of
the axis I managed to analyse the results of 114 patients' 123 medial and 9
patients' 9 lateral unicondylar arthroplasties. The sex ratio of women to
men in the medial side was 92/31 and in the lateral unicondylar type 8/1.
The average age was 67 years at the time of the operation and the follow-up
time varied between one and two years.
In the medial unicompartmental arthroplasties on the femur, on the average,
we corrected from the preoprative 93.2 (89-98) degrees to 97.5 (94-102)
degrees postoperatively, on the tibia from 82.2 (71-89) degrees to 86.6 (8092) degrees. According to these measurements the average correction on
the femur was 4.3 degrees and on the tibia 4.4 degrees. The correction of
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the lower extremity's varus position was the total of the above two angles,
8.7 degrees.
The result of the New Jersey knee score was on the average 87.9 (51-100)
points postoperatively.
I analysed in more detail the patients who had medial
unicompartmental knee prosthesis implantation (123 knees). Comparing
the functional results of the patients, who had 10 degrees or less correction,
to the patients, who had more than 10 degrees of correction, we did not get
any significant differences in the New Jersey knee scores (88.1 and 87.3 in
96 and 27 patients).
Further analysing the patient groups it becomes clear that there are
only 2 cases, in which the correction was more than 15 degrees (17 and 18),
in such cases the average result of the New Jersey knee score was 71.5
points. This result shows a significant difference compared to the patients',
who has had less than 15 degrees of correction (88.3 point) (table 2).
pain (30)
function (25)
movement (15)
deformity (12)
stability (10)
muscle pow.(8)
Summerised
N.J. (max.:100)
med. uni. </=150
26.7
20.5
12.8
11.6
8.9
7.8
med.uni.>150
24
12
12
11
6
6.5
88.3
71.5
lat unicond. F.P.
26
18.3
13.7
11.6
9.4
7.9
86.9
Table 2: The postoperative New Jersey (N. J.) knee scores compared in all
subgroups. The result of the patients, who had medial unicompartmental
knee arthroplasty with less than 15 or 15 degrees of correction, those
patients who had more than 15 degrees of correction and the lateral
unicompartmental cases.
In the patients who has had valgus gonarthrosis on the femur from
the average 101.7 (100-106) degrees we have corrected to 96.2 (94-98)
degrees, so the average correction on the femoral side was 5.5 (3-9)
degrees. Under the same deformation on the tibial side from the
preoperative 91.4 (88-95) degrees of valgus position, we have corrected to
postoperative 87.6 (84-89) degree. Thus the average correction on the tibial
side was 3.8 (0-11) degrees.
The average total correction (both the femur and the tibia) of angles on the
valgus knees was 9.3 degrees and only in one case was the correction more
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than 10 degrees, it was exactly 20 degrees, and this patient got the best New
Jersey knee score result, 98 points.
According to the New Jersey knee score we have achieved 86.9 points
following the lateral unicompartmental arthroplasties (table 2.).
5. DISCUSSION
In the recent years more and more attention turns to the
biomechanical analyses. These get a special role and importance in the case
of the knee joint as in this case prosthesis implantation means the
replacement of a complicated and complex joint. Other studies have also
dealt with cadaver knees and also with MRI measurements if possible.
I have to emphasize that we managed to create standard situations in case of
the X-ray measurements with careful positioning of the patients, and this
was also proven by statistical analyses .
In the case of cadaver knees I didn't manage to achieve repeated
measurements or have another person perform control measurements,
therefore in this case I used the method of the indirect proof. Comparing
the cadaver knees with the X-ray films using the two-sample t test, there
was no statistical difference between the two methods.
According to the above analyses, proved by two measuring methods, I do
think the prosthesis most used in our country for unicompartmental knee
arthritis, at the medial unicompartmental implantation causes the movement
path to shorten and also a backward shift on the tibial side. The shortening
and backward shift are increased by the cutting through ACL.
The above results seem to correlate with the clinical fact, we often
see in revision cases, the excavated area on the back part of the flat tibial
tray. This seems to be the result of the movement path's shortening and
backward shift. Similarly, in revision cases, following unicompartmental
knee prosthesis implantation the backward subluxation or luxation of the
femur on the tibia can be visible on side X-rays, in cases, where the ACL is
damaged or absent. In these cases the loosening of the tibial tray can also
be found during the reoperations.
I have already mentioned the importance of the anatomical and
mechanical axes in the introduction. The deviation from these axes causes
major changes in load distribution in the different compartments.
From the literature we know that following unicompartmental arthroplasty
lower succes rates can be achieved in the survival analyses, while with total
knee arthroplasty the range of motion and the patient's satisfaction is poorer
for higher allocated expenses.
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Under these circumstances, the question of how the correction of the axis
influences the functional results arises, how worthwhile it is, how much we
have to and how possible it is to make an effort to correct the deviation in
unicompartmental knee prosthesis implantation.
On the one hand, in the early postoperative months the patient's own
motivation, toleration of pain also largely influences the results. On the
other hand, a few years postoperatively early loosening can gradually occur,
which can also influence the results. In my opinion in the postoperative
first or second year the negative effects on the functional results are
minimal, the postoperatively achieved axis correction can be analysed
without any real changes.
Analysing the corrected axes we have altered the average
preoperative 93.2 degrees in 123 knees to 97.5 degrees of the anatomic axis
of the femur following medial unicompartmental knee replacement. On the
tibial side we have corrected the axis from the average 82.2 degrees to 86.6
degrees. According to the 6 degrees of anatomical valgus axis of the femur
and the 3 degrees of varus on the tibia, we have overcorrected the femoral
side with 1.5 degrees and under corrected on the tibial side with half a
degree. The differences from the anatomical axis are minimal, but we can
draw conclusions for operating techniques, as measurements were done on a
fairly great number of patients. I do think that it is important to take the
thickness of the femoral component into account for the correction angle,
especially if the preoperative valgus of the femur is close to normal. In this
case it is worth to make a groove with the oscillation saw in the femoral
condyle, which makes hiding the component possible and thus the varusvalgus angle correction will be less influenced.
Although the under correction on the tibial side is negligable, it draws
attention to the importance of certain operative technical steps. These
important techniques are the appropriate release of the collateral ligament
and the removal of the osteophytes on the affected component for the easier
release of the collateral ligaments and the more physiological axis.
In the lateral unicompartmental knee prosthesis implantation cases
the average of correction of 9 knees were from the 101.7 degrees to 96.2
degrees on the femoral side and from 91.4 to 87.6 degrees on the tibial side.
I'd like to emphasize the near physiological results, however it is obvious
that there was a much larger correction on the femur, than on the tibia (5.5
versus 3.8). This can partly be explained by the fact that the femoral
component was also purely build on the top of the sclerotic bone despite the
achieved angle being physiological, that is good.
Presenting the results I've demonstrated the analyses of the medial
unicompartmental knee prosthesis implantation devided into 10 degrees or
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less and 10 degrees plus correction groups and also the 15 degrees or less
and the 15 degrees plus groups. According to these comparisons we can
conclude that under 15 degrees the early functional results and thus the
patient's satisfaction do not depend on the correction angle. Analysing the
subgroups of the New Jersey knee score I did not find any significant
differencies in the range of movement, deformity, but there was significant
differences in the function, stability and muscle power categories. From the
pain point of view there was mild difference between the patient groups
with 15 degrees or less correction and the one with more than 15 degrees
(table 2.).
The poor results in the patient group with over 15 degree correction, in the
categories of function, stability and muscle power indicate the limitations of
the prosthesis and draw attention to the use of the proper indication critaria.
We can analyse the medial unicompartmental prosthetised patients
in the following way; we divide them considering the 96 degrees of femoral
valgus and 87 degrees of tibial varus normal alignment with +/- 2 degrees
of tolarence (this means 94-980 on the femur and 85-890 on the tibia). This
way in one of the groups, the patients had above tolerance results on both
sides, the next group of patients had one side in and the other one out of the
tolerance and in the third group patients had the angle out of both the femur
and the tibia from the above tolerance (table 3.). In these groups we did not
get any significant result. In my opinion this supports the idea, that the
prosthesis under a certain dimension can be implanted forgivingly as the
small difference from the anatomical angles doesn't influence the early
postoperative results.
It is difficult to separate valgus operated knee subgroups for
analysis and therefore to present statistical data, because of the few number
of patients. In spite of this the average total correction of the angle was
9.33 degrees and there was only one case, where the correction was greater
than 10 degrees, exactly 20 degrees, in which case I got the best New Jersey
knee score of 98 points in the lateral unicomparetmental group.
The
anatomical
femoral angle is
between 94-980, and
on the tibia 85-890
Only one of the
anatomical axes is
out of the 2-20
tolerance level
Both
of
the
anatomical axes are
out of the 2-20
tolerance level
N. J.
88.8
86.2
86.8
knee
scores
Table 3.: The postoperative anatomical axes in connection with the
functional results (New Jersey scores: N. J.)
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We achieved similar result following medial and lateral unicompartmental
knee replacement in the New Jersey knee scores, with mild variation of the
subgroups (table 2.: first and third row). These small differencies can be the
result of the different anatomical structures of the two compartments.
6. CONCLUSION
1) I have worked out two modelling techniques (cadaver and X-ray film),
in which I managed to measure the effect of the medial unicompartmental
knee arthroplasty results, especially on the tibia. I have concluded that my
measuring techniques can be compared to other results. As a conclusion of
my work I pointed out that the movement path has shortened and has
shifted backwards following medial unicompartmental knee arthroplasty.
On the X-ray films I have proved that we managed to create standardized
circumstances.
2., I've separately examined the role of the LCA. In deficiency of LCA the
path of the movement is shifted further back on the tibial plateau following
medial unicompartmental knee prosthesis.
3., Analysing the early results of the axis corrections I concluded that with
the particular prosthesis used, following medial or lateral unicompartmental
knee prosthesis implantation there is no significant difference in the early
functional results. This type of implant can be used with confidence in
medial unicompartmental knee arthroplasties up to 15 degrees of varus
deformation preoperatively, according to the early postoperative results. I
did not find any significant difference in the early functional results
between the 10 degrees or less and the more than 10 degrees corrected
groups, in our patient series. However in my opinion this does not mean
that this type of implant can be used for any kind of more than 10 degrees
of correction. It only supports the idea that under strict patient selection
criteria, as I detailed in the chapter about the prostheses in general and in
case of other indication conditions, with careful operating technique this
type of implant can even be used for 10-15 degrees of deviation.
4., From the operating technique point of view:
-It is important to properly position the implant. On the one hand
it means the proper burying of the front end of the femoral implant
and on the other the optimal 5-7 degree of posterior slope of the
tibial plateau.
-The above operating technical steps are important not only to
ensure proper gliding-rolling movement, but also to alleviate or
avoid the flexion contracture.
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-I draw the attention to the fact that not only the total (femoral plus
tibial) correction of angle is important, but also that these angles
have to correct the deformation properly on both the femur and the
tibia sides, to gain the horizontal plane of the knee joint
postoperatively.
-I emphasize that especially on the femoral side it is important to
calculate the correction of the deviation with the thickness of the
implant. This means that in the case of close to the normal femoral
valgus one must think of the proper burying of the implant on the
femoral condyle (we measured correction from 96 to 100 degrees
of valgus).
-It is also important to consider the tibial plateau's resection plane,
which influences the axis of the limb. However it is not the varusvalgus direction which will primarily determine this axis, but the
plateau's resection height, the tibial component height and in the
case of a varus knee the deliberation of the medial colateral
ligament and the removal of the osteophytes.
5. From prosthesis design point of view, I concluded two important changes
of the femoral part of the implant. On the one hand in the front part even
larger radius and forward elongated design, on the other hand in the back a
less sharp curved form would help the femoral component to move more
precisely under the physiological way.
6. Comparing the clinical experiencies to the result of my measurments in
the gliding-rolling movement, it seems to be supported by the tibial plateaus
found in revisions, as they have a special excavation in the back. This is the
result of the movement path's shortening and backward shift. Similarly side
X-rays of patients who has no LCA show that the femur is subluxated or
luxated backward on the tibial plateau.
7. To summerise I need to point out that the unicondylar knee arthroplasties
has its role in the treatment of certain degree of gonarthrosis. Better result
can be achieved in survival analyses with strict indication critaria and
careful operating technique. The importance of the unicondylar knee
arthroplasties lies in the greatest patient satisfaction, the successful survival
results when the patients are operated with strict selection criteria, the
practice of resection of the minimal and damaged parts only and thus to
achieve physiological biomechanics and at last if needed the better position
for revision.
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