Keywords
anterior cruciate ligament - soft-tissue graft - tibial tunnel - screw displacement
Introduction
Anatomy studies have led to a better understanding of the anatomical footprints of
the anterior cruciate ligament (ACL),[1] which contributed to the advance from non-anatomical to anatomical reconstructions.
The aim is to create a ligament that has the same bony attachments and course as those
of the native ligament, and to restore the stability and function of the injured knee.[2]
[3]
[4] Failure of an ACL graft may result from any combination of technical errors, biologic
causes, and trauma.[5] Suboptimal tunnel placement is a frequent problem among residents and even experienced
surgeons, despite the fact that the insertion points and intraoperative landmarks
for anatomical ACL reconstruction are well known.[6]
[7] This may predispose to early graft impingement and subsequent lack of mobility,
persistent instability, and failure.[8] Misplaced tibial tunnels may be addressed intraoperatively with interference screws
to shift the graft to a better position to avoid impingement against the lateral femoral
condyle.[9]
The purpose of the present study is to quantify the graft displacement of porcine
flexor tendons inside the tibial tunnel in terms of length and direction according
to four quadrant-specific screw locations.
Hypothesis
In a porcine ACL model, tibial interference screws constantly displace soft-tissue
grafts to the opposite side, regardless of the location of the screw.
Materials and Methods
An ex-vivo experimental study was performed on 28 porcine knees. The superficial digital
flexor muscle tendon was harvested from the lower portion up to its proximal insertion
in all specimens. The ACL ToolBox Instrument Set (Arthrex, Naples, FL, US) was used
to prepare the grafts and to perform the bone tunneling. Once harvested, all grafts
were cleaned from the remnant tissue and folded to create a double-band construct
with both ends stitched using #2 FiberWire (Arthrex) suture. All grafts were sized
to fit through the 9-mm block. The knees were mounted on a metal frame, and the tibial
plateaus were carefully dissected to preserve the anatomical tibial footprint of the
ACL. A tibial ACL marking hook was placed intentionally next the tibial ACL footprint
with the compass hand guide set to a 55° angle. A drill-tip guide pin was placed from
the anteromedial tibia to the articular surface, which was then drilled with a 9-mm
cannulated drill bit to create a misplaced tibial tunnel. The doubled grafts were
passed inside the tunnel, and a nitinol guide wire was positioned in a specific quadrant
to guide the 9-mm tibial interference polyether ether ketone (PEEK) screw. The grafts
were fixed with the tip of the screw at the level of the tibial articular surface
([Fig. 1]). One specimen suffered a tibial plateau fracture after the placement of the screw
of the anterior tunnel, and was excluded from the study. The remaining specimens were
divided into four groups according to the location of the tibial interference screw
in relation to the graft. In group A (N = 7), the interference screw was positioned anteriorly; in group P (N = 7), posteriorly; in group M (N = 7), medially; and in group L (N = 7), laterally to the graft. A millimetric ruler was placed at the tibial plateau,
which was photographed with an EOS T6 camera (Canon Inc., Ōta, Tokyo, Japan). All
images were digitalized, scaled to size, and taken from the same angle and length
at 3 specific moments: the bone tunnel without the graft, with the graft positioned
in the tunnel, and with the tip of the tibial screw reaching the tibial plateau ([Figs. 1] and [2]). The length and direction of the graft displacements were measured with Adobe Photoshop
CC 2019 (San Jose, CA, US) ([Fig. 2]). To assess the correct measurement values, the same procedure was repeated with
a digital caliper with a precision of 0.05 mm.
Fig. 1 Surgical technique. (A) Identification of the tibial ACL footprint. (B,C) Misplaced tibial tunnel that was drilled and measured. (D) Graft within the tibial tunnel. (E) Screw fixated at the articular surface. (F) Measurement of the diameter of the tibial tunnel after screw fixation.
Fig. 2 Schematic representation and measurements performed on digitalized images (Photoshop
CC 2019). (A) Graft within the tibial tunnel. (B) Tibial tunnel with graft and screw fixated at the articular surface. (C) Distance of the graft displacement (CX) measured in millimeters from the center
of the tibial tunnel (C) to the center of the graft (X). (D) The angle of displacement of the graft was measured in degrees as following: angle
between two arms, VX and VCZ, with center of the screw as the vertex (V). VX: line
from center of the screw (V) to the center of the graft (X). VCZ: line from the center
of the screw (V) passing through the center of the tunnel (C) to the tunnel perimeter
point (Z), which is 180° opposite to the entry location of the screw. This was repeated
for all groups.
Statistical Analysis
The Shapiro-Wilk test was performed, and it showed normal distribution among all specimens
(N = 28). The mean differences in length and direction of the graft displacements among
the four groups were analyzed through one-way analysis of variance (ANOVA). The statistical
analysis was performed using the Statistical Package for the Social Sciences (IBM
SPSS Statistics for Windows, IBM Corp., Armonk, NY, US) software, version 25.0, and
values of p ≤ 0.05 were considered statistically significant.
Results
Data describing the size of the tunnel before and after the fixation with screws,
as well as the length and direction of the graft displacement for each specimen in
each group are shown in [Table 1]. Graft displacement only occurred when the screw reached the tibial plateau, and
its mean length was similar in all four groups ([Fig. 3]): A – 4.4 mm; P – 4.6 mm; M – 4.5 mm; and L – 4.3 mm, with no statistically significant
differences (p = 0.894). The mean direction of the graft displacement was also similar among the
groups: A – 176° (standard deviation [SD]: 15.4°); P – 165° (SD: 16.6°); M –166° (SD:
12.1°); and L –169° (SD: 10.6°). No statistically significant differences were found
(p = 0.42).
Table 1
|
Group “A” (anterior)
|
|
|
Porcine tibia
|
Tunnel diameter without screw (mm)
|
Tunnel diameter with screw (mm)
|
Graft displacement (mm)
|
Graft direction (angle)
|
|
1
|
9
|
12.6
|
4.2
|
180.4°
|
|
2
|
9.2
|
13.6
|
4.7
|
160.3°
|
|
3
|
9.3
|
12.3
|
2.7
|
180.6°
|
|
4
|
9.3
|
11.9
|
4.5
|
180.4°
|
|
5
|
9.6
|
13.1
|
4.7
|
195°
|
|
6
|
9.5
|
13.2
|
5.1
|
186.6°
|
|
7
|
8.6
|
11.8
|
5
|
150.6°
|
|
AVERAGE
|
9.214
|
12.643
|
4.41
|
176.27°
|
|
Group “P” (posterior)
|
|
|
|
Porcine tibia
|
Tunnel diameter without screw (mm)
|
Tunnel diameter with screw (mm)
|
Graft displacement (mm)
|
Graft direction (angle)
|
|
1
|
9.1
|
11.9
|
3.8
|
188.4°
|
|
2
|
9.6
|
11.8
|
3.9
|
174.6°
|
|
3
|
9.1
|
11.3
|
3.5
|
169.6°
|
|
4
|
9.6
|
13.9
|
4.4
|
150.2°
|
|
5
|
9.7
|
13.8
|
5.8
|
137.5°
|
|
6
|
9.6
|
13.4
|
5.2
|
166.4°
|
|
7
|
8.9
|
13.8
|
5.6
|
168.4°
|
|
AVERAGE
|
9.37
|
12.843
|
4.60
|
165.01°
|
|
Group “L” (lateral)
|
|
|
|
Porcine tibia
|
Tunnel diameter without screw (mm)
|
Tunnel diameter with screw (mm)
|
Graft displacement (mm)
|
Graft direction (angle)
|
|
1
|
9.8
|
13.3
|
4.3
|
175.6°
|
|
2
|
8.8
|
11.5
|
3.8
|
155.6°
|
|
3
|
9.4
|
14.4
|
4.5
|
181.9°
|
|
4
|
8.7
|
12.2
|
4.7
|
154.9°
|
|
5
|
9.7
|
13.7
|
4.2
|
158.7°
|
|
6
|
9.7
|
11.5
|
4.7
|
155°
|
|
7
|
8.5
|
12.9
|
3.9
|
177.3°
|
|
AVERAGE
|
9.22
|
12.78
|
4.3
|
165.57°
|
|
Group “M” (medial)
|
|
|
|
Porcine tibia
|
Tunnel diameter without screw (mm)
|
Tunnel diameter with screw (mm)
|
Graft displacement (mm)
|
Graft direction (angle)
|
|
1
|
9.5
|
11.8
|
4.1
|
152.4°
|
|
2
|
9.6
|
12.1
|
4.5
|
166.6°
|
|
3
|
8.9
|
13.3
|
5.4
|
171.4°
|
|
4
|
10.2
|
13.6
|
5.4
|
167.2°
|
|
5
|
8.9
|
12.6
|
4.1
|
186.2°
|
|
6
|
8.8
|
12.8
|
3.7
|
161.7°
|
|
7
|
9.8
|
13.5
|
4.04
|
174.5°
|
|
AVERAGE
|
9.38
|
12.81
|
4.5
|
168.57°
|
Fig. 3 Mean distance and direction of the graft displacement measured in all four groups.
Discussion
The most important finding of the present study is that, regardless of the entry quadrant,
constant graft displacement to the opposite side was observed when the tibial screw
reached the articular surface. We were able to quantify the mean distance and direction
of the graft displacement using a quadrant-specific 9-mm PEEK screw inside a 9-mm
tibial tunnel; these measured diameters are frequently used in ACL reconstructions.
This distance could be use by the surgeon as a guide in case a misplaced tibial tunnel
is drilled. Nevertheless, the magnitude of this error cannot be predicted; hence,
it should be assessed case by case.
Parate and Chernchujit.[9] developed an arthroscopic technique to adjust the placement of the guidewire for
the interference screw in the tibial tunnel in anatomical single-bundle ACL reconstruction.
The authors[9] state that posterolateral placement of the tibial screw helps to push the graft
medially and anteriorly in the tibial tunnel to avoid impingement with the lateral
femoral condyle. As this is an arthroscopic technique, no precise information is given
regarding the magnitude and direction of the graft displacement. To date, no articles
have described the capability of tibial interference screws to correct misplaced tunnels
by shifting a soft-tissue graft toward a more anatomical location.
As aforementioned, suboptimal tunnel placement is a frequent problem among residents
and even experienced surgeons,[10] despite the fact that the insertion points and intraoperative landmarks for anatomical
ACL reconstruction are well known.[6]
[7] Most authors suggest[11] inspecting the tibial footprint before ACL reconstruction, as well as marking the
center of the planned tunnel with a radiofrequency ablation probe. The center of the
tibial tunnel should be located at 40% of the medial-to-lateral width of the interspinous
distance, in line with the posterior edge of the anterior horn of the lateral meniscus,
∼ 15 mm anteriorly to the posterior cruciate ligament. A tibial guide is set to 50°
and positioned with the aiming tip intraarticularly at the centrum of the ACL footprint.[12]
The present study helps the surgeon to predict where to place a screw inside a misplaced
9-mm tibial tunnel: a quadrant specific 9-mm interference screw fixed at the joint
line enables a constant displacement of 4.5 mm of the soft-tissue graft toward the
opposite side (169°; SD: 13.7°). Interestingly, this was only observed when the interference
screw reached the articular surface ([Fig. 3]). On the contrary, there was no displacement when the screw had not reached the
surface. This could be explained because an interference screw is only able to shift
a graft where both come into contact; hence, the screw will not have any influence
on graft displacement if it is placed bellow the intended level of correction, in
this case, the articular surface. This observation is a key factor in the present,
study and was observed in all specimens. It is well known that proximal tibial graft
fixation leads to more stable knees.[9]
[13] We also believe this observation is a key factor to achieve a constant graft displacement
when one needs to correct a misplaced tibial tunnel. Therefore, we suggest arthroscopic
visualization of the nitinol wire that will guide the tibial screw in a specific quadrant.
The screw is then inserted until visible and withdrawn until only the tip is at the
same level as the articular surface. The surgeon must take care of tensioning the
graft with their hand during screw insertion to prevent the graft from wrapping around
the screw. This can also be double-checked with an arthroscopic view from inside the
tibial tunnel, to confirm the desired position of the nitinol wire in relation to
the graft.
Another positive aspect of the present study is its reproducibility: since there were
no significant differences among the groups, comparisons could be performed. The present
study was performed in porcine knees mainly because of their wide availability, but
also because of the similar biomechanical properties and diameter of the porcine superficial
digital flexor tendon compared with human flexor tendons.[14]
[15]
[16] Graft fixation was performed with a PEEK interference screw because, compared with
titanium screws, they present excellent mechanical characteristics, biological compatibility,
and result in absence of metal artifacts on the MRI scan.[17] Regarding the surgical technique, we performed extraction drilling (ED) with a 9-mm
drill bit because the mean diameter of the porcine posterior flexor tendon was of
8.7 mm. A screw with a 9-mm diameter was used following the recommendations of the
manufacturer (Arthrex), who suggests using an implant with diameter as close as possible
to that of the graft. The same screw diameter was used for all the groups so the results
could be comparable. We have also decided to use 9-mm screws based on a 2013 study[18] which compared the ultimate failure load and cyclic displacement of different screws
for the fixation of soft-tissue grafts in a similar ACL porcine model. The authors[18] used 9-mm screws for 9-mm grafts inside 9-mm tunnels, with equal results for PEEK
screws compared with other types of screws. One of the drawbacks of the surgical technique
used in the present study is that serial dilation (SD) was not performed, and enlargement
of the tibial tunnel was observed in all the specimens after screw fixation ([Table 1]). Biomechanical studies have demonstrated that ED had a lower mean load to failure
for the graft, as well as increased tibial tunnel expansion and postoperative graft
migration at the tibial fixation compared with SD, but no functional differences were
found in a recent systematic review.[19] Although SD might reduce the expansion of the tibial tunnel, we believe this does
not alter the final objective of the present study, which was to quantify graft displacement
in terms of distance and direction.
Regarding the measurement techniques, previous studies[20] describe the use of a digital caliper. In the present study, graft size, tunnel
size before and after screw fixation, and graft displacement ([Figs. 2] and [3]) were also measured with a digital caliper with a precision of 0.05 mm. These values
were very similar to those observed with digitalized images set to scale using Adobe
Photoshop CC 2019, which we decided to use because it offers more accurate tools to
measure the angular displacement of the graft.
Screw positioning may also have an effect on the orientation of the graft. Parate
and Chernchujit[9] mention that the placement of posterolateral tibial screws may have an effect on
graft obliquity, and Mall et al.[21] demonstrated that more vertical grafts on MRI are associated with greater anterior
tibial translation on Lachman testing. We agree with the aforementioned authors that
ACL graft obliquity is particularly sensitive to tibial tunnel placement and can influence
knee stability. Since femoral tunnel drilling was not performed, graft obliquity could
not be evaluated in the current study. We believe a future cadaveric study with anatomical
femoral and tibial ACL reconstruction should be performed to determine if a quadrant-specific
tibial screw has any influence on graft obliquity and knee stability.
Conclusions
Regardless of the entry quadrant, a constant mean graft displacement of 4.5 mm to
the opposite side was observed when the tibial screw reached the articular surface.
Clinical relevance: non-anatomically placed ACL soft-tissue grafts can be corrected
intraoperatively with the use of quadrant-specific tibial interference screws. Nevertheless,
we cannot predict the magnitude of this error in every case of failed tibial tunnel
drilling; hence, it should be assessed case by case.
