Keywords
ACL - tunnel widening - tunnel enlargement
Bone tunnel enlargement is a well-established phenomenon occurring predominantly within
the first 3 months following anterior cruciate ligament (ACL) reconstruction surgery.[1]
[2] The highest percentage of change in femoral and tibial tunnel size occurs within
the first 6 weeks after surgery.[1] However, tunnel enlargement has been reported up to 2 years postoperatively.[1]
[3]
[4] The incidence of tunnel enlargement is particularly related to hamstring autografts
with large reported variability ranging from 25 to 100% in femoral tunnels and 29
to 100% in tibial tunnels.[5]
[6]
[7]
[8] First attributed to allografts[3]
[9] and bone-tendon-bone (BTB) grafts,[10]
[11] tunnel enlargement related to hamstring autografts was first described by ĹInsalata
and Harner in the late nineties.[2] Clatworthy et al proposed a multifactorial etiology of tunnel enlargement with a
biochemical component after performing suspensory fixation in both hamstring and BTB
grafts, finding a higher incidence of tunnel enlargement in hamstring grafts.[12] Faunoe and Kaalund reported more distinct tunnel enlargement in cortical fixation
compared with transverse pin fixation of hamstring grafts, concluding that the graft
fixation site in relation to the joint is crucial in the development of tunnel enlargement.[13] The exact etiology of tunnel enlargement however remains unclear and is believed
to be a multifactorial process including both mechanical and biological factors.[10]
[12]
[14]
[15]
[16]
[17]
[18]
[19] Mechanical factors include graft tunnel-motion, especially in tunnel malposition;
drill-related bone necrosis; and aggressive rehabilitation.[7]
[9]
[10]
[20]
[21]
[22]
[23]
[24] Biochemical factors include synovial fluid propagation and cytokine-induced osteolysis,
eventually aggravated by absorbable fixation implants.[3]
[9]
[10]
[23] The clinical relevance of tunnel enlargement is uncertain. Although the majority
of studies did not reveal a correlation between tunnel enlargement and clinical outcome,[2]
[3]
[8]
[9]
[12]
[17]
[20]
[23]
[25]
[26]
[27] some studies have recognized tunnel enlargement to be an early sign of graft failure.[28] However, a clinically important issue is that revision surgery is complicated by severe tunnel enlargement, eventually
making the two-stage ACL revision surgery necessary.[29] Previous studies have focused on the correlation between bone tunnel enlargement and surgical technique, graft
choice, and rehabilitation.[2]
[8]
[10]
[17] To our knowledge, the correlation between bone tunnel enlargement and original bone
tunnel diameter has not been elucidated.
Purpose and Hypothesis
The purpose of this study was to determine whether bone tunnel enlargement after ACL
reconstruction with hamstring autograft measured on computed tomography (CT) is dependent
on original tunnel diameter established during primary ACL reconstruction surgery.
As both mechanical and biological causes of tunnel enlargement may theoretically be
dependent on graft-tunnel contact area and bone–tendon interface, we hypothesized
that smaller diameter tunnels are more susceptible to tunnel enlargement than larger
tunnels.
Methods
Patients
All patients with accessible CT scanning of femoral and tibial bone tunnels after
ACL reconstruction were identified. As CT is used as a part of preoperative revision
evaluation, a cohort of 122 consecutive patients, who were scheduled for ACL revision
surgery at the Aarhus University Hospital between 2013 and 2016, was identified. Of
these patients, the study included 56 patients with primary ACL reconstruction using
hamstring autograft and accessible primary operative reports and a new CT scan of
femoral and tibial bone tunnels as part of the preoperative ACL revision evaluation.
These inclusion criteria enabled CT-based evaluation of tunnel enlargement after hamstring
autograft ACL reconstruction. Medical records including operative reports were assessed
to identify original femoral and tibial bone tunnel diameter established during primary
ACL reconstruction (range: 6–9 mm). Furthermore, patient demographics and graft fixation
methods were recorded ([Table 1]).
Table 1
Graft fixation methods in relation to original bone tunnel diameter established during
primary ACL reconstruction (range: 6–9 mm)
|
Original bone tunnel diameter (mm)
|
6
|
6.5
|
7
|
7.5
|
8
|
8.5
|
9
|
|
Number of patients (n)
|
1
|
2
|
15
|
3
|
16
|
1
|
17
|
|
Primary femoral ACL graft fixation
|
|
Cortical suspension
|
1
|
2
|
13
|
3
|
14
|
1
|
14
|
|
Transverse pin fixation
|
0
|
0
|
2
|
0
|
2
|
0
|
2
|
|
Interference screw
|
0
|
0
|
0
|
0
|
0
|
0
|
1
|
|
Primary tibial ACL graft fixation
|
|
Nonabsorbable screw
|
1
|
2
|
15
|
3
|
5
|
1
|
9
|
|
Absorbable screw
|
0
|
0
|
0
|
0
|
3
|
0
|
3
|
|
Nonspecified screw
|
0
|
0
|
0
|
0
|
8
|
0
|
5
|
Abbreviation: ACL, anterior cruciate ligament.
CT Assessment
Femoral and tibial bone tunnel enlargement was assessed by CT scanning (mean time
from ACL reconstruction to CT tunnel measurement = 40.8 months) using the traditional
two-dimensional (2D) CT method.[30] The transosseus diameter of femoral and tibial tunnels was measured at each tunnel
midpoint in coronal, sagittal, and axial CT image planes using a linear measuring
tool ([Fig. 1]).
Fig. 1 Two-dimensional (2D) computed tomography (CT) measuring method.[30] Bone tunnels are assessed in coronal, sagittal, and axial CT image planes.
Statistics
Mean tunnel diameter values were calculated and analysis of the correlation between
original tunnel diameter and bone tunnel enlargement was investigated using regression
analysis.
Results
Tunnel enlargement from the original tunnel diameter to CT measured, follow-up diameter
for both femoral and tibial bone tunnels is presented in [Table 2]. For femoral tunnels, original 7-mm bone tunnels showed a mean tunnel enlargement
of +0.15 mm (p = 0.576). Original 8-mm bone tunnels showed a mean tunnel enlargement of −0.003 mm
(p = 0.987), while 9-mm original bone tunnels showed a mean tunnel enlargement of −0.16 mm
(p = 0.574). For tibial tunnels, original 7-mm tibial bone tunnels showed a mean tunnel
enlargement of +1.93 mm (p = 0.0001). Original 8-mm bone tunnels showed a mean tunnel enlargement of +1.38 mm
(p = 0.0001), while original 9-mm bone tunnels showed a mean tunnel enlargement of +0.83 mm
(p = 0.002). As seen in [Fig. 2], mean tibial bone tunnel enlargement is significantly and inversely dependent on
the original tibial bone tunnel diameter with a correlation coefficient of −0.55 (p = 0.007). Thus, every additional increase (mm) in diameter regarding the original
tibial bone tunnel reduces the extend of tibial tunnel widening by 0.55 mm. There
was no significant correlation between tunnel enlargement and the elapsed time from
primary ACL reconstruction to CT follow-up measurement (mean = 40.8 months; range = 7–139
months; femoral tunnels, p = 0.2; tibial tunnels, p = 0.06). There was no significant correlation between tunnel enlargement and patient
age.
Table 2
Tunnel enlargement presented as change in original bone tunnel diameter
|
Original tunnel diameter
|
Femoral mean bone tunnel enlargement (CI, p-value)
|
Tibial mean bone tunnel enlargement (CI, p-value)
|
|
7 mm
|
+0.15 mm; (CI: −0.4–0.7, p = 0.576)
|
+1.93 mm (CI: 1.4–2.4, p = 0.0001)
|
|
8 mm
|
−0.003 mm (CI: −0.42–0.3, p = 0.987)
|
+1.38 mm (CI: 1.1–1.7, p = 0.0001)
|
|
9 mm
|
−0.16 mm (CI: −0.7–0.4, p = 0.574)
|
+0.83 mm (CI: 0.3–1.3, p = 0.002)
|
Abbreviation: CI, confidence interval.
Fig. 2 Regression analysis of the correlation between mean tibial bone tunnel enlargement
and original tibial bone tunnel diameter (coefficient correlation: −0.55; confidence
interval [CI]: −0.944–156; p = 0.007).
Discussion
The primary finding of the this study was that tunnel enlargement of tibial bone tunnels
after hamstring ACL reconstruction was inversely correlated to the original tunnel
diameter, with small diameter tunnels showing more excessive tunnel enlargement than
larger tunnels. Second, femoral bone tunnels did not demonstrate any significant tunnel
enlargement. The dependency of bone tunnel enlargement on original bone tunnel diameter
has not been described before. Clatworthy et al proposed a multifactorial etiology
of tunnel enlargement with a biochemical component after performing suspensory fixation
in both hamstring and BTB grafts and finding a higher incidence of tunnel enlargement
in hamstring grafts.[12] Interestingly, the authors mentioned an evident difference in graft size distribution
with hamstrings ranging from 6 to 10 mm and BTB grafts ranging from 9 to 13 mm. Considering
the results of this study, it seems possible that the differences in original tunnel
diameter might have contributed to the finding of more tunnel enlargement for hamstring grafts in
the study of Clatworthy et al, as hamstring grafts classically have the smallest original
tunnel diameters in comparison to grafts with bone blocks. The fact that femoral tunnels
in comparison to tibial tunnels do not show significant tunnel enlargement has been
reported before.[12] A possible explanation could be drill-related bone necrosis, which theoretically
is less marked at the femoral site, as the femoral bone-reamer contact area is irrigated
with arthroscopic fluid in contrast to the tibial site. Also, the femoral condylar
bone has a higher density than proximal tibial bone, which could make femoral bone
more resistant to tunnel enlargement after ACL reconstruction. Furthermore, tibial
tunnel enlargement may be related to the biomechanical stress caused by the interference
screw, which theoretically may be more distinct in smaller bone–tendon interfaces.
However, a multifactorial etiology of tunnel enlargement must be assumed. The majority
of studies regarding tunnel enlargement after ACL reconstruction surgery have focused
on the correlation between bone tunnel enlargement and surgical technique, graft choice,
fixation method, and aggressiveness of rehabilitation.[2]
[8]
[10]
[17]
[31]
[32] Considerably fewer studies have focused on the potential biochemical causes of tunnel
enlargement including the interaction of synovial fluid with the bone–tendon interface.[33] The graft-tunnel contact area may represent a key point in the etiology of tunnel
enlargement. Small diameter tunnels with less bone–tendon interface may be more susceptible
for drill-related bone necrosis and biomechanical stress, especially in tunnel malposition.
In addition, inflammatory cytokines in the synovial fluid may affect the tibial bone–tendon
interface more excessively than femoral tunnels, as gravity tends to direct synovial
fluid into the tibial bone tunnel. This could explain why femoral tunnels did not
show significant tunnel enlargement. ACL reconstruction with graft diameters less
than 8 mm in diameter has been shown to be associated with higher revision rates.[29]
[34] Insufficient graft material has been proposed as a potential cause. The results
from this study suggest an additional cause for these clinical findings, as small
diameter ACL reconstructions will have a higher proportion of tunnel enlargement that
could result in graft fixation failure. The small and inhomogeneous patient cohort
comprising different graft fixation methods represents a limitation of this study.
Furthermore, all accessible CT scans represented patients who had failure of their
ACL reconstruction, and therefore, could represent a population with altered biomechanics.
Studies with larger cohorts are needed to investigate the correlation between original
bone tunnel diameter and bone tunnel enlargement. The results of this study present
a new perspective on bone tunnel enlargement etiology, as small diameter bone tunnel
diameter may represent an unrecognized factor favoring bone tunnel enlargement. However,
further studies are needed to revise the findings of this study.
Conclusion
The results of this study indicate that tibial bone tunnel enlargement following ACL
reconstruction is significantly and inversely dependent on the original tibial bone
tunnel diameter. Every additional increase (mm) in diameter regarding the original
tibial bone tunnel reduces the extend of tibial tunnel widening with 0.55 mm. The
contributing factors remain unclear and need to be further investigated.
