Keywords
ACL - tibial slope - LTPS - MBA - FMIA - meniscal bone angle
The anterior cruciate ligament (ACL) enables stable knee kinematics by limiting tibial
rotation and anterior tibial translation. ACL reconstruction (ACLR) is performed to
improve knee stability and shows overall satisfactory results and low revision rates.[1]
[2] Numerous studies have investigated factors which are associated with the etiology
of ACL injury and ACLR failure to reduce ACLR failure rates.[3]
[4]
[5]
[6]
[7] In addition to neuromuscular control and other physiological factors, the geometrical
shape of the knee joint including the posterior tibial slope and the meniscal–cartilage
interface have been shown to affect the risk of ACL injury.[8]
[9]
[10] However, the influential strength of these geometrical factors and the influence
on the risk of ACL failure are not yet fully understood. Christensen et al found that
the tibial posterior slope is increased in patients with early graft failure after
ACLR; a six-degree increase of tibial cartilage slope resulted in a 10 times higher
risk of ACL graft failure.[11] Sturnick et al found that reduced meniscal bone angle (MBA) is associated with increased
risk of ACL injury in females.[10] These findings could support the concept that increased tibial slope increases ACL
graft strain, while a functional lateral meniscus contributes to restrain against
tibial anterior translation and rotation and consequently may reduce graft strain.[12]
[13]
[14]
[15]
[16]
[17] Consequently, the integrity and geometrical shape of the lateral meniscus are of
paramount importance for knee stability and may affect ACLR outcome.[18] The susceptibility of ACL graft failure is associated with numerous dependent and
independent geometrical factors including notch width, meniscal slope, meniscal height,
cartilage slope, and MBA.[10] Lateral tibial posterior slope (LTPS) and MBA represent two independent geometrical
factors which may affect the risk of ACL failure and which may eventually potentiate
or neutralize each other. LTPS and MBA can be assessed on magnetic resonance imaging
(MRI) on the same slide with high reliability.[10]
[19] The purpose of this study was to investigate whether reduced MBA is associated with
increased risk of ACL graft failure and if the LTPS–MBA ratio represents a feasible
method for the assessment of ACLR failure risk.
Material and Methods
After approval by the local ethics committee, a cohort of 1,480 consecutive patients
who underwent ACL hamstring reconstruction surgery by a single surgeon between August
2000 and May 2013 was reviewed. In the hospital database, 86 ACLRs had been identified
as failures by means of clinical failure and MRI with or without subsequent ACL revision
surgery. Clinical failure was defined as ACL graft rupture or insufficiency with consecutive
subjective instability and abnormal laxity upon clinical examination. Laxity was not
consequently quantified by means of an arthrometer. All patients were followed prospectively
with active evaluation 1 year postoperatively and hereafter by need-based appointments.
Patients were hereby advised to contact the clinic in case of complications including
recurrent instability with or without preceding relevant trauma. Hamstring grafts,
suspensory femoral fixation, and a tibial interference screw were used in all cases.
Exclusion criteria ([Fig. 1]) comprised nonaccessible MRI (20 patients) and the presence of MRI-verified lateral
meniscus lesions as these lesions could theoretically alter the MBA (12 patients).
Fifty-four patients (32 males; 22 females) with ACL graft failure were finally included
in this study. Twenty-eight patients had undergone transportal ACLR, while 26 patients
had undergone transtibial ACLR. Patients were matched 1:1 by age, sex, graft, fixation
method, and surgical technique with 54 control participants, who had undergone ACLR
with a minimum of 4 years of follow-up without signs of graft failure. Patients were
hereby actively evaluated 1 year postoperatively and hereafter advised to contact
the clinic in case of complications including recurrent instability with or without
preceding trauma. A total of 108 patients were anonymized and randomized for blinded
assessment. MRI (minimum 1.5 Tesla) was used to determine the lateral tibial slope
based on the technique described by Hudek et al.[9] The first step of this technique consists in finding the central sagittal image
in which the tibial attachment of the posterior cruciate ligament and the intercondylar
eminence is seen ([Fig. 2]). Subsequently, two circles are placed in the tibial head. A cranial circle which
touches the anterior, posterior, and cranial cortex and a caudal circle which touches
the anterior and posterior cortex. The center of the caudal circle is hereby positioned
on the circumference of the cranial circle. The line connecting the centers of both
circles is defined as the MRI longitudinal axis of the tibia and is propagated through
the sagittal MRI series. In the following step, the axial anatomical center of the
lateral plateau is identified and a tangent to the lateral plateau is drawn which
connects the uppermost even part between the superior–anterior and posterior cortices.
The angle between the orthogonal line to the MRI longitudinal axis and the tangent
to the lateral plateau is defined as the LTPS. The MBA was measured as described by
Sturnick et al[10] between a tangent to the superior meniscal surface and the tangent to the subchondral
tibial bone on the same slide (see [Fig. 3]). The measuring method described by Hudek et al[9] has previously been validated showing excellent reliability (Typical Error [TE] ± 1.4°
for interobserver reproducibility and ± 1.2° for intraobserver reproducibility; Correlation
Coefficiant [CC] 0.80 for intraobserver and 0.77 for interobserver reproducibility).
The measuring technique described by Sturnick et al[10] has previously been validated showing excellent reliability (intraclass correlation
coefficient intraobserver 0.9).[20] LTPS and MBA were assessed on MRI after ACL injury and before primary ACLR. All
measurements were conducted by a single blinded observer on a radiology suite computer
with the necessary software (OsiriX). Data are presented as mean values ± standard
deviation and were investigated using logistic regression and receiver operating characteristic
curve estimation by an independent professional statistician. For all analyses, a
p-value of < 0.05 was considered significant.
Fig. 1 Flowchart describing the criteria used for selection of patients included in this
study.
Fig. 2 The measuring technique described by Hudek et al. In the first step, the central
sagittal image is identified in which the tibial attachment of the posterior cruciate
ligament (PCL) and the intercondylar eminence is seen. Subsequently, two circles are
placed in the tibial head. A cranial circle which touches the anterior, posterior,
and cranial cortex and a caudal circle which touches the anterior and posterior cortex.
The center of the caudal circle is hereby positioned on the circumference of the cranial
circle. The line connecting the centers of both circles is defined as the magnetic
resonance imaging (MRI) longitudinal axis of the tibia and is propagated through the
sagittal MRI series. The anatomical center of the lateral tibial plateau is identified
on axial slides.
Fig. 3 A tangent to the lateral plateau is drawn which connects the uppermost even part
between the superior–anterior and posterior cortices. The angle between the orthogonal
line to the magnetic resonance imaging (MRI) longitudinal axis and the tangent to
the lateral plateau is defined as the lateral tibial posterior slope. The meniscal
bone angle is measured as described by Sturnick et al[10] between a tangent to the superior meniscal surface and the tangent to the subchondral
tibial bone on the same slide.
Results
In this cohort, 39 patients (36%) showed a LTPS–MBA ratio of under 0.27 which was
associated with a 28% risk of ACL failure, while 33 patients (31%) showed a ratio
exceeding 0.42 which was associated with an 82% risk of ACL failure. Odds of ACL failure
increased by 22.3% per degree of decreasing MBA (odds ratio [OR], 1.22; 95% limits,
1.1–1.34). The ACLR failure group showed a significantly reduced mean MBA of 20.5° ± 3.9°
(range, 12.7°–28.7°) compared with the control group with 24.5° ± 4.6° (16.3°–32.6°;
p < 0.001). Regarding the entire study population, no significant association was found
between LTPS and the risk of ACL graft failure (OR, 1.11; 95% limits, 0.96–1.29).
In the transportal ACL failure group, the odds of ACL failure increased by 34.9% per
degree of increasing LTPS (OR, 1.34; 95% limits, 1.01–1.79), while no significant
correlation was found between LTPS and the risk of graft failure in the transtibial
ACL failure group. The results are summarized in [Tables 1]
[2]
[3]. The entire ACL failure group including transportal and transtibial ACLR failures
showed an increased LTPS of 7.9° ± 2.8° (range, 2.2–15.5) compared with the control
group with 7.1° ± 2.8° (range, 3.2°–16°; p = 0.15), which was not significant. When investigating subgroups, the transportal
ACLR failure group showed a significantly increased mean LTPS of 8.58° compared with
the control group with 7.16° (p = 0.028), see [Table 4]. In the isolated transportal ACLR group (n = 28), a LTPS–MBA ratio of under 0.27 was associated with a 12% risk of ACL failure
(34% of patients), while a LTPS–MBA ratio exceeding 0.47 was associated with a 98%
risk of ACL failure (29% of patients). LTPS and MBA of the transportal and transtibial
ACLR groups are presented in [Table 5]. No significant correlation was found between MBA and LTPS (p = 0.5).
Table 1
LTPS and MBA values of the ACL failure group and matched control group
|
|
LTPS
|
MBA
|
|
Controls (n = 54)
|
Mean ± SD (min–max)
|
7.13° ± 2.43° (3.2°–16°)
|
24.5° ± 4.62° (16.3°–32.6°)
|
|
Failures (n = 54)
|
Mean ± SD (min–max)
|
7.86° ± 2.81° (2.2°–15.5°)
|
20.53° ± 4.14°(12.7°–28.7°)
|
|
p-Value
|
Differences, means
|
0.15
|
< 0.001
|
|
Odds ratio
|
Risk of failure
|
1.11 (11.4% per > unit)
|
1.22 (22.3% per < unit)
|
Abbreviations: ACL, anterior cruciate ligament; LTPS, lateral tibial posterior slope;
MBA, meniscal bone angle; SD, standard deviation.
Table 2
Results for a LTPS–MBA ratio cut-point of 0.27
|
Controls
|
Failures
|
|
|
|
≤ 0.27 (n = 39/108)
|
28
|
11
|
Sensitivity
|
80%
|
|
> 0.27 (n = 69/108)
|
26
|
43
|
Specificity
|
52%
|
|
N
|
54
|
54
|
Negative predictive value
|
72%
|
|
|
|
Positive predictive value
|
62%
|
Abbreviations: LTPS, lateral tibial posterior slope; MBA, meniscal bone angle.
Table 3
Results for a LTPS–MBA ratio cut-point of 0.42
|
Controls
|
Failures
|
|
|
|
≤0.42 (n = 75/108)
|
48
|
27
|
Sensitivity
|
50%
|
|
> 0.42 (n = 33/108)
|
6
|
27
|
Specificity
|
89%
|
|
N
|
54
|
54
|
Negative predictive value
|
64%
|
|
|
|
Positive predictive value
|
82%
|
Abbreviations: LTPS, lateral tibial posterior slope; MBA, meniscal bone angle.
Table 4
LTPS of the transportal and transtibial ACLR failure group and control group
|
LTPS failures
|
LTPS controls
|
p-Value
|
|
Transportal ACLR (n = 26)
|
8.58
|
7.16
|
=.028
|
|
Transtibial ACLR (n = 28)
|
7.2
|
7.1
|
=.9
|
Abbreviations: ACLR, anterior cruciate ligament reconstruction; LTPS, lateral tibial
posterior slope.
Table 5
MBA of the transportal and transtibial ACLR failure group and control group
|
MBA failures
|
MBA controls
|
p-Value
|
|
Transportal ACLR (n = 26)
|
25.0
|
20.7
|
<0.001
|
|
Transtibial ACLR (n = 28)
|
24.0
|
20.3
|
0.003
|
Abbreviations: ACLR, anterior cruciate ligament reconstruction; MBA, meniscal bone
angle.
Discussion
The primary finding of this study was that reduced MBA is associated with increased
risk of ACL graft failure, regardless of ACLR technique and graft positioning. Second,
increased LTPS was associated with significantly increased risk of ACL graft failure
in the transportal failure group, while no significant association was found in the
transtibial failure group. The results of this study suggest that the tibial slope
has a higher impact on transportal ACLR compared with transtibial ACLR failure risk,
while the MBA effects transportal and transtibial ACLR similarly; the reasoning for
this discrepancy remains unknown and needs to be further investigated. However, a
possible explanation could be that slope-related graft strain may be potentiated by
nonisometric graft positioning, as in transportal ACLR, where the femoral tunnel had
been placed central in the ACL footprint. When examining the entire ACL failure group,
36% of the patients showed a LTPS–MBA ratio of under 0.27 and this was associated
with a 28% risk of ACL failure, while 31% of the patients showed a ratio exceeding
0.42 which was associated with an 82% risk of ACL failure. The given cut-off points
were derived from a logistic model for feasible sensitivity and specificity as well
as negative and positive predictive values. No correlation between MBA and LTPS was
found.
Recent studies have underlined the contribution of the lateral meniscus to rotational
knee stabilization, especially in ACL deficient knees.[8]
[18]
[21]
[22] Getgood et al suggested that the lateral meniscus should be regarded as a part of
the anterolateral capsulomeniscal complex stabilizing rotation in conjunction with
the ACL.[8] Interestingly, Sturnick et al found that decreased MBA is associated with primary
ACL injury in females,[10] which supports the assumption that the geometrical shape of the lateral meniscus
affects ACL strain forces in normal knees. Furthermore, the geometrical shape of the
lateral meniscus may become more influential in ACL reconstructed knees where rotational
stability is not fully restored. This may not only have implications for meniscal
treatment procedures but also for the assessment of ACL failure risk depending of
geometrical factors of the knee joint. Recent studies have shown that increased tibial
slope is associated with increased risk of ACL injury and ACLR failure.[17]
[19]
[23] Increased anterior tibial translation is thought to be the primary mechanism for
this finding.[19] As the position of the menisci is dependent on the underlying surface, it seems
conceivable that increased LTPS levels out the femoral meniscal interface angle (FMIA)
without substantially affecting MBA, while increased MBA steepens the FMIA without
affecting LTPS ([Fig. 4]). This could be the explanation why increased MBA may theoretically neutralize increased
LTPS ([Fig. 4]) and why the combination of increased LTPS and decreased MBA may be associated with
increased risk of graft failure ([Fig. 5]). ACL rupture results in subluxation of the tibiofemoral joint. Increased lateral
tibial slope may increase the acceleration of this event, which normally is counteracted
by the posterior meniscal horn ([Fig. 5]). A dysfunctional posterior meniscal horn may not decelerate this event sufficiently
resulting in higher ACL strain and eventually ACL rupture. Increased MBA supports
the deceleration of the pivoting event and may therefor reduce the likelihood of ACL
rupture. Further studies are needed including weight-bearing MRI to investigate the
effect of axial loading on the FMIA. In addition, it remains unclear, to what extend
passive stabilization as by the menisci is accountable for deceleration of the femur
in pivoting events in contrast to active muscle stabilization, which may be of greater
importance.
Fig. 4 To maintain the same functional femoral meniscal interface angle (FMIA), the meniscal
bone angle (MBA) needs to increase if lateral tibial posterior slope (LTPS) increases.
Fig. 5 (A) Increased meniscal bone angle (MBA) and reduced lateral tibial posterior slope (LTPS)
compared with (B), resulting in a reduced femoral meniscal interface angle (FMIA). The reduced FMIA
may be more effective for deceleration of the femur (circle) in pivoting maneuvers.
In conjunction with active muscular stabilization, the meniscus may impede tibiofemoral
subluxation and subsequent anterior cruciate ligament (ACL) injury. Increased LTPS
may accelerate the femur in pivoting maneuvers and may be best counteracted by a meniscus
with increased MBA.
However, the susceptibility of ACL injury and ACLR failure is multifactorial including
the surgical technique, neuromuscular conditions, patient age, level of function,
timing of return to sport, compliance to rehabilitation protocols, acquired concomitant
injuries, and structural anatomy of the knee joint.[3]
[4]
[5]
[6]
[7]
[23]
[24]
[25]
[26]
[27]
[28]
[29] Some factors may also be mutually dependent, e.g., young patient age has been shown
to be associated with increased tibial cartilage slope which itself is associated
with higher incidence of concomitant meniscal injury and early graft failure.[11] An increasing body of literature is indicating that the cartilage and meniscal slopes
may play a more important role for knee kinematics than subchondral slopes. Christensen
et al found that the tibial posterior cartilage slope is increased in patients with
early graft failure after cruciate ligament reconstruction,[11] while Sturnick et al found that reduced MBA is associated with increased risk of
ACL injury in females.[10] In addition, an association between meniscal slope and increased risk of ACL injury
has been described.[30] However, the reliability of some measuring techniques has been questioned. Meniscal
slope is measured by connecting the peaks of the anterior and posterior horn of the
meniscus and it seems conceivable that the geometry of the posterior meniscal horn
may have been the primary mechanism for this previously described association.[10] Furthermore, cartilage slope is measured as a tangent to an eventually convex cartilaginous
center of the lateral tibiofemoral compartment, which may impede measurement accuracy
and reproducibility. Sturnick et al depicted statistically significant relationships
between several geometrical intra-articular features including a correlation between
MBA and the meniscus–cartilage height.[10] The assessment of these geometrical features using MRI is controversially discussed.
Even though high measures of reliability for several slope measurement methods are
reported in the literature,[9]
[10] there is disagreement regarding the actual slope values[19]; key slope cut-off points which are associated with significantly increased risk
of ACL failures are therefore difficult to determine. In this study, we have focused
on two geometrical features without substantial correlation that may affect graft
strain and which can be measured on the same MRI slide with high reliability.[10]
[19] Furthermore, the ratio of LTPS and MBA may be of greater importance than the actual
slope value, as it might be conceivable that MBA may neutralize or potentiate LTPS
and vice versa.
An increasing body of research is emphasizing the contribution of the lateral meniscus
to sagittal knee stability[18]
[22] and the importance of meniscal integrity for better ACLR outcomes. Consequently,
meniscal repair procedures including the transtibial technique for meniscal root repairs
have been popularized.[21] However, the exact contribution of the lateral meniscus depending on the geometrical
shape and integrity is not yet fully understood. Cho et al found that a simple tear
of the lateral meniscus does not increase localized pressure in porcine knees when
the meniscofemoral ligament is intact[31]; this may explain why these tears are rarely symptomatic in human knees.[11] Future studies are needed to deepen the knowledge regarding the function of the
lateral meniscus and meniscofemoral ligaments as well as the role of suturing techniques
for the maintenance of the MBA. The results of this study support the concept that
the lateral meniscus has an important role regarding knee stabilization and that the
integrity and geometrical shape may affect ACLR outcomes.
The results of this study emphasize that geometrical features of the knee joint including
tibial slope as well as the cartilage–meniscal interface may affect the risk of ACL
failure. It is not intended to reveal actual LTPS and MBA cut-off points for clinical
practice. Slope correcting osteotomies should be reserved for special cases.[32]
[33] However, a standardized method for assessment of LTPS and MBA may be useful for
the assessment of ACL failure risk and may have implications for graft choice or the
use of extra-articular stabilizing procedures. This study has limitations. Patients
were actively evaluated 1 year postoperatively and hereafter advised to contact the
clinic in case of complications including recurrent instability with or without preceding
trauma. The true number of failures beyond the first postoperative year is unknown
and probably underestimated, as not all patients with graft failure are assessed.
It is conceivable that patients with reduced tibial slope and graft failure might
refrain from reassessment as they do not experience substantial instability. This
could represent a potential bias. Other limitations include the small number of patients
and the risk of confounding as other factors may influence the results of this study
including patient age, sex, activity level, other geometrical features of the knee
as cartilage slope and height, the condition of meniscal tissue, physiological factors
such as neuromuscular control and quadriceps-dominant deceleration, as well as hormonal
factors. MBAs have been measured on conventional MRI without axial loading of the
lower limb which theoretically could alter the MBA. Further studies are needed to
evaluate if preoperative assessment of ACLR failure risk based on the geometrical
shape of the knee joint is a useful procedure.
Perspective
A growing body of research is indicating that the tibial slope and the geometry of
the tibiofemoral meniscal–cartilage interface affect the risk of ACL injury.[10]
[11] Increased tibial slope (LTPS) may accelerate pivoting kinematics while the menisci
may be of paramount importance for deceleration of these events. To our knowledge,
this is the first study to combine the tibial slope and MBA to assess the risk of
ACL failure. In the future, MRI-based assessment of geometrical features of the knee
joint prior to ACLR surgery may help to identify patients at high risk of ACLR failure.
This may have implications for patient counseling and the indication of additional
extra-articular stabilizing procedures.
