Abstract
Background: Graft reconstructions of the lateral collateral
ligament, popliteus tendon, and popliteofibular ligament are frequently
performed in conjunction with a reconstruction of the posterior cruciate
ligament to restore knee stability. The purpose of this study was to determine
the femoral fixation sites resulting in the optimum isometry of popliteus
tendon, popliteofibular ligament, and lateral collateral ligament grafts in a
knee with a reconstruction of the posterior cruciate ligament.
Methods: Relative length changes (isometry measurements) were
recorded between sutures fixed at femoral grid points and appropriate fibular
or tibial graft tunnel sites; sites resulting in the least change in suture
length as the knee was moved from 0° to 90° of flexion were identified
as optimum isometric points. Bone blocks of Achilles tendon grafts were fixed
with the midpoint of the tissue's leading edge adjacent to the optimum
isometric point (optimum placement). Isometry measurements were repeated with
a lateral collateral ligament graft placed in a fibular tunnel and with
popliteus tendon and popliteofibular ligament grafts alternately placed in
appropriate tibial and fibular tunnels. The graft isometry measurements were
then repeated with the bone block centered over the femoral footprint of the
lateral collateral ligament or popliteus tendon.
Results: For all reconstructions, there was no difference between
the relative length changes of the suture placed at the optimum isometric
point and the relative length changes of the graft with an optimally placed
bone block. The mean location of the optimally placed bone-block center of the
lateral collateral ligament graft was within 1.85 mm of the mean center of the
footprint of the lateral collateral ligament; the mean graft isometry
measurements with the optimally placed bone block were not significantly
different from those with the bone block centered over the lateral collateral
ligament footprint. The mean optimally placed bone-block center of the
popliteus tendon and popliteofibular ligament reconstructions was 11 mm
anterior and 2.7 mm proximal to the center of the popliteus tendon footprint.
The mean relative length changes of the popliteus tendon and popliteofibular
ligament grafts with the bone block optimally placed were <0.9 mm and
<1.2 mm, respectively; the means with the popliteus tendon and
popliteofibular ligament bone blocks centered over the popliteus tendon
footprint were 3.7 mm and 5.0 mm, respectively.
Conclusions: A popliteus tendon or popliteofibular ligament
reconstruction with the bone block centered over the femoral footprint of the
popliteus tendon was highly non-isometric. If the graft were fixed at 30°
of flexion, it would elongate approximately 4 mm when the knee was extended to
0° and possibly stretch out.
Clinical Relevance: We found suture isometry to be a good indication
of graft isometry. In situ measurements of relative suture-length changes at
the time of surgery may be helpful in determining a femoral fixation site that
will result in graft isometry.
The lateral collateral ligament, popliteus tendon, and popliteofibular
ligament have been identified as major stabilizing structures of the
posterolateral corner and are often disrupted with an injury to the posterior
cruciate
ligament1-3.
Prior biomechanical studies have shown that forces in the posterior cruciate
ligament can increase dramatically after sectioning of the posterolateral
structures, especially during application of external tibial torque or a varus
moment4,5.
Therefore, reconstruction of the posterolateral structures is important to
restore knee stability and to protect a posterior cruciate ligament graft. To
prevent either stretch-out or slackening of a posterolateral graft, its
femoral tunnel should be located at an isometric point such that there is a
minimal change in graft length throughout a 90° range of motion. This
ensures that pretension applied to the graft will be maintained at a constant
level as the knee is flexed, thereby preventing abnormal varus and external
rotation laxity of the reconstructed knee.
While numerous surgical procedures for reconstructing the posterolateral
structures have been
described6-11,
recent emphasis has been placed on procedures that reproduce the normal
anatomy of these structures. Recommendations for anatomic reconstructions vary
but generally include placement of the proximal fixation tunnels at the
centers of the femoral footprints of the lateral collateral ligament and the
popliteus
tendon12-16
and the distal fixation tunnels at the anatomic attachment sites on the
fibular head and the proximal-lateral aspect of the tibia, respectively.
Although it is common surgical practice to drill femoral tunnels at the
centers of the footprints of the native popliteus tendon and lateral
collateral ligament for application of posterolateral grafts, the isometry of
posterolateral reconstructions has not been studied, to our knowledge.
Measurements of relative length changes of sutures fixed at grid points on the
lateral femoral condyle allow construction of an "isometry map"
that can be used to select an isometric femoral point for a specific graft
reconstruction. Relative length changes of a graft fixed at that location can
then be compared with the suture measurements and with measurements of grafts
fixed at the centers of anatomic femoral footprint sites.
The goal of this study was to determine the femoral sites that would result
in the most isometric lateral collateral ligament, popliteus tendon, and
popliteofibular ligament graft reconstructions. We hypothesized that centering
a lateral collateral ligament graft over the femoral footprint of the lateral
collateral ligament and centering popliteus tendon and popliteofibular
ligament grafts over the center of the popliteus tendon footprint would
provide the least relative changes in graft lengths over a 90° range of
motion.
Fifteen fresh-frozen knees that we used in another study on posterolateral
knee
reconstruction17
were used in the current study. The mean age of the donors was 35.1 years
(range, seventeen to sixty-five years); thirteen were male, and two were
female. The tibia and femur were sectioned at the mid-part of the shaft and
scraped clean of soft tissue to within 10 cm of the joint line. The bone ends
were potted in cylindrical molds of polymethylmethacrylate acrylic cement for
gripping in the test fixtures. The test apparatus maintained the tibia in a
level position while the femur was flexed through a 90° range of
motion18. Tibial
rotation was measured with a goniometer at each flexion angle studied, and the
tibia was free to seek its own axis of rotation. Full extension (0° of
flexion) was defined as the tibiofemoral angle resulting from application of a
2.5-N-m extension moment to the knee.
The femoral attachments of the lateral collateral ligament and popliteus
tendon were removed, and the centers of each footprint were identified. The
lateral epicondyle was identified as the lateral-most projection of the
lateral femoral condyle after all soft tissue was removed. A 2 × 2-cm
coordinate grid was superimposed on the lateral femoral condyle. The origin of
the coordinate system was located at the center of the femoral footprint of
the popliteus tendon (Fig. 1).
With use of a template, twenty-one holes were drilled into the femur. Four
columns, each with five holes, were created with the holes in adjacent rows
and columns spaced 5 mm apart. An additional hole was drilled 5 mm distal to
the popliteus tendon footprint, near the articular cartilage margin. The
columns were oriented parallel to the long axis of the femur
(Fig. 1). Grid coordinates for
the center of the lateral collateral ligament footprint and for the lateral
epicondyle were recorded.
For the reconstructions of the lateral collateral and popliteofibular
ligaments, tunnels (6 mm in diameter) were drilled into the fibula at the
respective anatomic attachment sites on the fibular head. For the popliteus
tendon reconstruction, a tunnel (7 mm in diameter) was drilled into the tibia
1 cm inferior to the lateral tibial plateau.
In each specimen, the posterior cruciate ligament was cut and reconstructed
with a single-bundle, 10-mm patellar tendon tibial inlay graft placed in the
anatomic footprint of the anterolateral bundle of the posterior cruciate
ligament. The graft was tensioned to restore anterior-posterior laxity at 200
N to within 1 mm of that in the intact knee at 90° of flexion.
A low-stretch synthetic suture, tied to a nail placed in a femoral grid
hole, was passed through the center of a cylindrical metal plug placed in a
graft tunnel and through a split clamp fixed to the tibia
(Fig. 2). With the knee at
0° of flexion, a dial caliper was used to measure a baseline length
between a forceps fixed to the suture and the tibial split-clamp
(Fig. 2). The knee was then
incrementally flexed to 10°, 30°, 45°, 70°, and 90°. At
each flexion angle, changes in suture length were recorded relative to the
length recorded at 0° of flexion.
Suture isometry testing was then conducted. Since length-change
measurements were sensitive to varus-valgus and tibial rotations, it was
necessary to reproduce the position of the tibia relative to the femur at each
knee flexion angle tested. This was done by manually holding the tibia at the
midpoint of internal-external rotation while applying a slight manual
compressive force along the axis of the tibia to ensure contact at both
condyles. A spring scale was used to apply a 27-N force to the end of the
suture line to maintain constant tension during the measurements. An increase
in relative length with knee flexion corresponded to slackening of a graft
fixed at full extension. Isometry testing was repeated with the suture placed
at each grid point and through the lateral collateral ligament, popliteus
tendon, and popliteofibular ligament graft tunnels. For each graft tunnel, the
femoral grid point that resulted in the least change in suture length
throughout a 90° range of motion was identified as the optimum isometric
point.
Achilles tendon grafts, sized to fill a 6-mm-diameter fibular tunnel (for
the reconstructions of the lateral collateral and popliteofibular ligaments)
or a 7-mm-diameter tibial tunnel (for the reconstruction of the popliteus
tendon), were used for all posterolateral reconstructions. The soft-tissue end
of the graft was interwoven with a low-stretch synthetic suture with use of a
whip stitch. A 1 × 1-cm calcaneal bone block was fixed into a square
mortised hole chiseled into the lateralpart of thefemur. The bone block was
positioned such that the midportion of the leading edge of tissue would lie
adjacent to the optimum isometric point
(Figs. 3-A and 3-B). Relative
length changes of the suture line (attached to the end of the graft placed in
a fibular or tibial tunnel) were measured as described above. Additional graft
isometry tests were performed with the bone blocks centered on the femoral
footprints of the lateral collateral ligament and the popliteus tendon. For
these tests, the old mortised recess was filled with polymethyl-methacrylate,
and a new recess was created, centered over the footprint. The same graft was
used for the reconstructions of the popliteus tendon and the popliteofibular
ligament. Clinically, both popliteus tendon and popliteofibular ligament
grafts are commonly fixed at the center of the popliteus tendon
footprint16.
Suture isometry maps were made to illustrate the trend of suture excursions
across the isometry grid for each reconstruction
(Fig. 1). To do this, the mean
maximum relative change in suture length from 0° to 90° was determined
for each grid point by averaging the absolute values for the maximum suture
excursions for all specimens. This value was assigned a positive or negative
sign on the basis of the direction of the change in the majority of specimens.
Positive numbers indicate relative suture lengthening with increasing knee
flexion (corresponding to slackening of a graft fixed at both ends with the
knee at 0° of flexion), and negative numbers indicate relative suture
shortening with knee flexion (corresponding to tightening of a graft fixed at
both ends at 0° of flexion).
Mean curves of relative length changes for each test condition were
calculated by averaging the values for all specimens at each flexion angle. A
repeated-measures analysis-of-variance model was used to compare mean relative
length changes between test conditions at each flexion angle; pairwise
comparisons were made with use of the Student-Newman-Keuls test. The test
conditions consisted of (1) a suture line fixed at the optimum isometric
point, (2) a graft fixed with the leading edge of the bone block centered at
the optimum isometric point (optimal placement), and (3) a graft fixed with
the center of the bone block at the footprint center. The level of
significance was set at p < 0.05.
Isometry Maps for Reconstructions of the Lateral Collateral Ligament,
Popliteus Tendon, and Popliteofibular Ligament
The mean maximum relative changes in suture length for all reconstructions
followed a consistent pattern across specimens, becoming more positive from
anterior to posterior and from distal to proximal on the grid (Tables
I,
II, and
III). In general, the grid
locations in which <80% of the specimens differed in sign were those with
smaller magnitudes of relative length change (<4.23 mm).
Femoral Locations of the Epicondyle, Lateral Collateral Ligament, and
Popliteus Tendon
The mean location of the epicondyle was approximately 10.6 mm proximal and
3.50 mm anterior to the center of the popliteus tendon footprint
(Figs. 3-A and 3-B). The mean
location of the center of the footprint of the lateral collateral ligament was
approximately 11.50 mm proximal and 0.5 mm anterior to the center of the
popliteus tendon footprint (Fig.
3-A); this placed it approximately 3.0 mm posterior and 0.85 mm
proximal to the mean epicondyle center. The mean location of the optimum
isometric point for the reconstruction of the lateral collateral ligament was
approximately 4.8 mm distal and 1.8 mm anterior to the mean center of the
footprint of the lateral collateral ligament
(Fig. 3-A). The mean location
of the bone-block center of the lateral collateral ligament graft (optimally
placed) was approximately 0.17 mm proximal and 1.85 mm anterior to the center
of the footprint of the lateral collateral ligament
(Fig. 3-A).
The mean location of the optimum isometric point for the reconstructions of
the popliteus tendon and the popliteofibular ligament was approximately 6 mm
anterior and 2.7 mm proximal to the center of the popliteus tendon footprint
(Fig. 3-B). The mean location
of the bone-block center of the popliteus tendon or popliteofibular ligament
(optimally placed) was approximately 11 mm anterior and 2.7 mm proximal to the
center of the popliteus tendon footprint
(Fig. 3-B).
Mean Curves of Relative Length Change
A suture fixed at the optimum isometric point for a reconstruction of the
lateral collateral ligament underwent a mean relative length change of less
than ±1 mm between 0° and 90°
(Fig. 4). The mean relative
changes in the length of a lateral collateral ligament graft with the bone
block optimally placed (with the leading edge of the graft tissue centered at
the optimum isometric point) were less than ±0.9 mm and were not
significantly different from the measurements of the sutures placed at the
optimum isometric point (Fig.
4). The mean relative graft-length changes with the bone block
centered over the footprint of the lateral collateral ligament reached a
maximum of 2.5 mm and were not significantly different from those with optimal
bone-block placement (Fig.
4).
A suture fixed at the optimum isometric point for a popliteus tendon
reconstruction underwent a mean relative length change of less than
±1.2 mm between 0° and 90° of flexion
(Fig. 5). The mean relative
length changes of a popliteus tendon graft with the bone block optimally
placed were less than ±0.9 mm and were not significantly different from
the suture measurements at the optimum isometric point
(Fig. 5). The mean relative
length changes with the bone block centered over the popliteus tendon
footprint reached a maximum of 3.7 mm and were significantly greater than
those with optimal bone-block placement
(Fig. 5).
A suture fixed at the optimum isometric point for a popliteofibular
ligament reconstruction underwent a mean relative length change of less than
±1.2 mm between 0° and 90°
(Fig. 6). The mean relative
graft-length changes with the bone block optimally placed were less than
±1.2 mm and were not significantly different from suture measurements
at the optimum isometric point (Fig.
6). The mean relative graft-length changes with the bone block
centered over the popliteus tendon footprint reached a maximum of 5.0 mm and
were significantly greater than those with optimal bone-block placement at
flexion angles of >10° (Fig.
6).
Research related to graft isometry has been focused largely on cruciate
ligament reconstructions, while little (if any) attention has been given to
the posterolateral structures of the knee. Any graft used for ligament
reconstruction must be optimally placed to effectively stabilize the joint
throughout the range of motion and prevent possible stretching of the repair
over time. Our data demonstrate that the relative lengths of lateral
collateral ligament grafts and of popliteus tendon and popliteofibular
ligament grafts fixed at the footprints of the lateral collateral ligament and
the popliteus tendon, respectively, increase with knee flexion and that most
of the increase occurs between 0° and 45° of knee flexion (Figs.
4,
5, and
6). This means that grafts
tensioned and fixed with the knee in full extension would slacken as the knee
is flexed. However, if a posterolateral graft is tensioned and fixed with the
knee in a flexed position, the force in the graft would increase when the knee
was extended to 0°. For example, our data show that if a popliteofibular
ligament reconstruction centered over the femoral footprint of the popliteus
tendon was fixed and tensioned with the knee in 30° of flexion, it would
lengthen an average of 4.2 mm when the knee was extended to 0°. The
resulting increase in graft force could cause the graft to permanently stretch
or fail during the initial healing period and increase external rotatory
laxity. Clinically, posterolateral grafts are commonly tensioned and fixed at
30°15. Thus,
identification of graft fixation sites that produce an isometric graft has
important implications for the long-term success of these reconstructions.
The maximum suture-length changes averaged across specimens at each grid
point (Tables I,
II, and
III) provide information
regarding isometry trends for fixation points on the femoral condyle. The
trend of increasing positive values suggests that grafts placed more posterior
and proximal to the center of the popliteus tendon footprint will slacken with
flexion (when fixed at 0°). Such locations would produce greater
elongation of the graft (from 30° to 0° of knee flexion) if the graft
were tensioned and fixed at 30°. While the pattern was consistent for all
specimens, it should be noted that suture measurements at some grid locations
were not in the same direction in all specimens. These inconsistencies may be
attributed to anatomical variations in femoral geometry across the specimens
used for this study.
Clinically, posterolateral grafts are fixed in bone tunnels with use of
interference screws positioned superior or anterior-superior to the graft
tissue13,14,16,19.
This method of fixation would not have been satisfactory for our studies,
since we were testing multiple femoral graft locations. An interference screw
creates permanent damage to the tunnel wall during insertion and removal, thus
eliminating the possibility of drilling adjacent holes for repeated testing.
Therefore, we chose to use 1 × 1-cm bone blocks fixed into a mortised
recess in the lateral femoral condyle. By trial and error, we found that the
match between suture measurements of the optimum isometric points and graft
measurements was best when the bone block was placed with its leading edge
centered over the optimum isometric point. For all reconstructions, the mean
relative suture-length changes measured with the suture fixed at the optimum
isometric point did not significantly differ from those measured with the bone
block optimally placed (on the basis of the optimum isometric point). It is
possible that use of an interference screw to secure graft tissue in a drilled
tunnel would constrain the tissue to one edge of the tunnel and the tissue
would function in a manner similar to that of the leading edge of tissue in
the bone blocks used in this study. If this were true, the center of the bone
block would represent the center of the tunnel during a reconstruction.
However, it is not known if placement of a femoral graft tunnel (relative to
the optimum isometric point) would differ from placement of the mortised bone
blocks used in this investigation.
On the basis of the optimum isometric point, the center of the bone block
of the lateral collateral ligament reconstruction was placed approximately
1.85 mm anterior to the center of the footprint of the lateral collateral
ligament. Relative graft-length changes with this placement were <0.9 mm
over the flexion range. Clinically, <2 mm of excursion of the graft through
the range of motion is thought to be
acceptable20,21.
As reconstruction based on the optimum isometric point placed the center of
the bone block in close proximity to the footprint of the lateral collateral
ligament, our data support the use of the femoral footprint of the lateral
collateral ligament for femoral placement of a graft used to reconstruct the
lateral collateral ligament.
In contrast, reconstructions of the popliteus tendon and the
popliteofibular ligament that were based on the optimum isometric point placed
the center of the bone block 2.7 mm proximal and 11 mm anterior to the
popliteus tendon footprint. When the bone-block centers were placed over the
center of the popliteus tendon footprint, the mean graft excursions of the
popliteus tendon and popliteofibular ligament reconstructions were
approximately +4 mm and +5 mm, respectively. These data suggest that the
traditional femoral placement of popliteus tendon and popliteofibular ligament
grafts at the center of the popliteus tendon footprint is not the best
choice.
The reason why use of the femoral footprint of the popliteus tendon will
not result in popliteus tendon and popliteofibular ligament grafts is readily
apparent. The popliteus originates as a tendon and ends as a muscle. Since
there is no bone-to-bone connection (as there is, for example, for the lateral
collateral ligament), there is no reason to expect that a graft between the
femur and tibia (the popliteus tendon graft) or between the femur and fibula
(the popliteofibular ligament graft) would be isometric when it is fixed at
the femoral footprint of the popliteus tendon. Contraction of the popliteus
muscle can adjust for any abnormalities in isometry throughout a range of
motion, a feature that is not possible with static grafts used for
reconstruction.
Much care was taken to determine the precise anatomic locations of the
lateral epicondyle and footprint centers. Previous work has demonstrated the
variability between the identification of the lateral epicondyle on knee
specimens and identification with computed tomography
imaging22,
suggesting that there could have been error in the determination of the
location of the lateral epicondyle in our study. However, despite the
variability in the relative locations of ligament insertions, on the average
we found the center of the lateral collateral ligament footprint to be 0.85 mm
proximal and 3.0 mm posterior to the lateral epicondyle of the femur, a
finding that is consistent with those of previous
reports23,24.
The distal location of the insertion of the popliteus tendon was an average of
11 mm from the center of the footprint of the lateral collateral ligament,
which was also consistent with observations in previous
reports23,24.
With regard to the anterior-posterior location of the popliteus tendon
footprint in reference to the lateral collateral ligament, our data are
consistent with those of Brinkman et
al.23 but are
contrary to the work of LaPrade et
al.24. These
discrepancies are likely due to the more variable anterior-posterior location
of the popliteus tendon
insertion24.
In the present study, attention was focused on varying the locations of the
femoral origins of posterolateral grafts. It was not possible to vary the
locations of the fibular insertions of the lateral collateral ligament and
popliteofibular ligament grafts to a substantial degree because of the
relatively small size of the fibular head. For this reason, tunnels were
drilled at appropriate anatomic footprint centers on the fibula. Since the
native popliteus has a muscular attachment on the posterior part of the tibia,
it was not possible to identify a static insertion site corresponding to a
bone-to-bone attachment, as was done for the insertions of the lateral
collateral ligament and the popliteofibular ligament on the fibular head.
Therefore, the tibial tunnel for the popliteus tendon was located at the
customary surgical point approximately 1 cm below the tibial plateau, near
where the tendinous portion of the popliteus muscle passes.
When applying these data to clinical practice, one must consider the
practical implications of the procedures used in this study. There was no
difference between the mean relative length changes of sutures fixed at the
optimum isometric point and the mean relative graft-length changes measured
after optimal placement of the bone block (on the basis of the optimum
isometric point). This suggests that isometry testing with use of sutures
could be clinically useful for determining the placement of the femoral graft
tunnel. However, we took great care in placing the tibia in neutral rotation
with dual condylar contact at each knee flexion angle. Suture isometry
measurements performed with the patient on the operating table may not exactly
duplicate our laboratory measurements.
In conclusion, our results suggest that placement of a lateral collateral
ligament reconstruction at a point near the footprint of the lateral
collateral ligament will result in the least amount of graft excursion through
a 90° range of motion. Placement of a popliteus tendon or popliteofibular
ligament reconstruction at a point 11 mm anterior and 2.7 mm proximal to the
center of the popliteus tendon footprint will result in the least amount of
graft excursion between 0° and 90°. ?
Note: Graft tissues for this study were provided by the
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