Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
DOI 10.1007/s00167-012-1941-6
KNEE
The role of the tibial slope in sustaining and treating anterior
cruciate ligament injuries
Matthias J. Feucht • Craig S. Mauro •
Peter U. Brucker • Andreas B. Imhoff
Stefan Hinterwimmer
•
Received: 8 August 2011 / Accepted: 23 February 2012 / Published online: 7 March 2012
Ó Springer-Verlag 2012
Abstract
Purpose A steep tibial slope may contribute to anterior
cruciate ligament (ACL)-injuries, a higher degree of
instability in the case of ACL insufficiency, and recurrent
instability after ACL reconstruction. A better understanding of the significance of the tibial slope could improve the
development of ACL injury screening and prevention
programmes, might serve as a basis for individually
adapted rehabilitation programmes after ACL reconstruction and could clarify the role of slope-decreasing osteotomies in the treatment of ACL insufficiency. This article
summarizes and discusses the current published literature
on these topics.
Methods A comprehensive review of the MEDLINE
database was carried out to identify relevant articles using
multiple different keywords (e.g. ‘tibial slope’, ‘anterior
cruciate ligament’, ‘osteotomy’, and ‘knee instability’).
The reference lists of the reviewed articles were searched
for additional relevant articles.
Results In cadaveric studies, an artificially increased
tibial slope produced an anterior shift of the tibia relative to
the femur. While mathematical models additionally demonstrated increased strain in the ACL, cadaveric studies
have not confirmed these findings. There is some evidence
that a steep tibial slope represents a risk factor for non-
M. J. Feucht P. U. Brucker A. B. Imhoff S. Hinterwimmer (&)
Department of Orthopaedic Sports Medicine, Technical
University Munich, Ismaninger Str. 22, 81675 Munich, Germany
e-mail: Stefan.Hinterwimmer@lrz.tu-muenchen.de
C. S. Mauro
Department of Orthopaedic Surgery, University of Pittsburgh
Medical Center, UPMC St. Margaret, 200 Delafield Road,
Suite 4010, Pittsburgh, PA 15215, USA
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contact ACL injuries. MRI-based studies indicate that a
steep slope of the lateral tibial plateau might specifically be
responsible for this injury mechanism. The influence of the
tibial slope on outcomes after ACL reconstruction and the
role of slope-decreasing osteotomies in the treatment of
ACL insufficiency remain unclear.
Conclusion The role of the tibial slope in sustaining and
treating ACL injuries is not well understood. Characterizing
the tibial plateau surface with a single slope measurement
represents an insufficient approximation of its threedimensionality, and the biomechanical impact of the tibial
slope likely is more complex than previously appreciated.
Level of evidence IV.
Keywords Tibial slope Anterior cruciate ligament Osteotomy Non-contact injury Knee biomechanics
Introduction
While laxity of the knee joint is influenced by various
structures including the cruciate and collateral ligaments,
the menisci, the joint capsule and the surrounding muscles
and tendons, it is also affected by the articular surface
geometry of the femur and tibia [14, 19, 34, 62, 74, 89].
Recently, the contribution of the tibial plateau geometry,
especially its posterior inclination, the so called tibial
slope, has been the focus of various investigations. It is
believed that the tibial slope has a direct influence on
sagittal plane laxity and therefore contributes to the loading
behaviour of the anterior cruciate ligament (ACL) [4, 11,
15, 19, 31, 49, 99].
Commonly, the tibial slope is measured on lateral
radiographs and is defined as the angle between a line
perpendicular to the longitudinal axis of the tibia and a
Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
tangent along the medial tibial plateau [28, 51]. However,
no uniform method exists to measure the tibial slope
because different longitudinal tibial axes are currently used
[12, 19, 51, 68, 82, 107]. Besides anatomical axes, such as
the proximal tibial anatomical axis, the proximal fibular
anatomical axis or a tangent along the anterior and posterior
tibial cortex [12, 82, 107] (Fig. 1), the mechanical axis of
the tibia has also been used [107] (Fig. 2). These different
axes are not parallel, and values of the tibial slope within
one tibia differ depending on the axis used [12, 102, 107].
Using the proximal tibial anatomical axis, values of 7° to
13° have been considered to be physiological [9, 19].
From a biomechanical view, the tibial slope produces an
anteriorly directed shear force component when a compressive tibiofemoral load or a quadriceps muscle force is
applied to the knee joint, resulting in an anterior translation
of the tibia (ATT) relative to the femur (Fig. 3). This
mechanism has been observed in cadaver models [89, 100]
as well as in living subjects [5, 19, 21]. In a radiographic in
vivo study by Dejour and Bonnin [19], a steeper tibial slope
resulted in a significantly greater amount of ATT in both
ACL-deficient and ACL-intact knees. Since the ACL is the
primary restraint against ATT [14, 26], the tibial slope may
therefore affect the in situ forces of the ACL. Several
authors have demonstrated increased ACL strain or ACL
rupture after isolated or combined axial tibiofemoral
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compression and quadriceps loading [22, 25, 63, 65, 72,
105]. This mechanism has been attributed to the tibial
slope.
Fig. 2 Tibial slope measurement based on the mechanical axis of the
tibia. The mechanical axis is defined as the line connecting the
midpoint of the tibial plateau and the tibial plafond
Fig. 1 Different anatomical axis used to measure the tibial slope.
1: proximal fibular anatomical axis (PFAA); 2: posterior tibial cortex
(PTC); 3: proximal tibial anatomical axis (PTAA); 4: anterior tibial
cortex (ATC)
Fig. 3 Biomechanical consequence of the tibial slope. Due to the
tibial slope, tibiofemoral compressive load (red arrow downwards)
and quadriceps muscle force (red arrow upwards) lead to an
anteriorly directed shear force resulting in an anterior translation of
the tibia relative to the femur (green arrow)
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This relationship between the tibial slope, ATT and
ACL strain might be of clinical relevance for several reasons. In ACL-intact subjects, a steep tibial slope might
represent a risk factor for non-contact ACL injury because
of increased ATT and ACL strain during axial tibiofemoral
compression and quadriceps loading. A better understanding of the role of the tibial slope in ACL injuries might help
to improve ACL injury screening and prevention programmes and consequently reduce the incidence of these
injuries. In ACL-deficient patients, a steeper tibial slope
might correlate with greater instability due to an increased
ATT. The tibial slope could therefore serve as a measurable parameter to identify patients who would most likely
be unable to cope with ACL insufficiency. During early
weightbearing after ACL reconstruction, a steep tibial
slope might place increased load on the healing graft and
fixation material and potentially increase the risk of early
elongation or acute failure. Similarly, late failure might
occur due to repetitive overloading and subsequent elongation of the graft. Improved knowledge about the effect of
the tibial slope on the graft after ACL reconstruction might
serve as a basis for individually adapted postoperative
rehabilitation programmes. Since high tibial osteotomy
(HTO) facilitates the modification of the tibial slope, the
question arises of whether a slope-decreasing osteotomy
represents a therapeutic option to treat sagittal plane laxity
due to ACL insufficiency or to prevent graft failure in
patients with a steep tibial slope undergoing ACL
reconstruction.
The biomechanical impact of the tibial slope, however,
might be even more complex, as characterization of the
tibial plateau surface geometry with a single slope on lateral radiographs likely represents an insufficient approximation of its three-dimensionality [36]. Because of
substantial differences between the slopes of the medial
and lateral compartment [16, 35, 47, 50, 54, 58, 107], the
surface geometry of the tibial plateau may also influence
rotational movements between tibia and femur [7, 69, 90, 103].
Furthermore, the mechanical relevance of the bony tibial
slope might be influenced by other parameters such as the
meniscal slope, which is defined as the angle between a
line perpendicular to the longitudinal axis of the tibia and a
tangent along the most anterior and posterior part of the
medial or lateral meniscosynovial border [50], and the
depth of the concavity of the medial tibial plateau [36].
Therefore, magnetic resonance imaging (MRI) has been
used in several studies to measure the slope of the medial
and lateral tibial plateaus separately and to investigate
other factors that might influence the mechanical effect of
the tibial slope [36, 37, 47, 55, 68, 96, 98].
While the importance of the tibial slope is well accepted
in the treatment of posterior and posterolateral knee
instabilities [2, 3, 29, 30, 77, 79], its impact on the native
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Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
ACL, anterior knee laxity and knee function following
ACL reconstruction is not well understood. The purpose of
this review article is therefore to summarize and discuss the
current published literature relevant to these topics.
Influence of artificial changes in tibial slope
on ACL strain and knee laxity
The effect of an artificially increased tibial slope on the
biomechanics of the knee joint has been investigated in
mathematical models [49, 64, 86, 87] and cadaveric studies
[1, 24, 31, 67].
Using a two-dimensional mathematical knee model, Liu
and Maitland [64] demonstrated an increase in ATT from
7.5 to 17.8 mm in ACL-deficient knees during walking
when the tibial slope was increased from 4° to 12°.
Shelburne et al. [87] used a computer model to examine
how changes in tibial slope affect knee biomechanics
during activities of daily living. Tibial slope was altered in
1° increments up to a maximum change of 10°. Increasing
the tibial slope resulted in a nearly linear increase of
anterior tibial shear force, ATT and ACL loading during
standing, squatting and walking. The effect of the tibial
slope on ACL force was most noticeable during gait: The
ACL force increased by 16 N for each degree increase in
tibial slope. Similar calculations were obtained by Shao
et al. [86]. In their mathematical model, which used electromyography, joint position and force plate data as inputs,
increasing the tibial slope from 4° to 8° resulted in a greater
amount of ATT, anterior tibial shear forces and ACL
loading during gait.
In a human cadaver model, Agneskirchner et al. [1]
increased the tibial slope in 5° increments via an anterior
opening wedge HTO up to a maximum change of 20°.
During simulated flexion–extension motion, ATT
increased with higher values of tibial slope. The maximum
ATT of 7.2 mm was observed when the tibial slope was
increased by 20° and the joint was positioned in 30° of
flexion. In addition, increasing the tibial slope caused a
superior translation of the tibial plateau relative to the
femoral condyles, with a maximum of 4.1 mm noted in full
extension. Giffin et al. [31] studied the effect of a 5-mm
anterior opening wedge HTO in 10 cadaveric knees under 3
different loading conditions of 200 N axial compression,
134 N anterior-posterior (a.-p.) tibial load or combined
200 N axial and 134 N a.-p. loads using a robotic testing
system. Following HTO, tibial slope increased by an
average of 4.4°. Compared to preoperative measurements,
a significant relative anterior shift of the tibial resting
position (the position of the knee at which all external
forces are minimized) was seen throughout the range of
knee motion with a maximum anterior shift of 3.6 mm
Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
noted in full extension. Under isolated axial compression, a
significant increase in ATT of 2 mm occurred at 30° and
90° of knee flexion. No significant changes of total a.-p.
translation were observed at any knee flexion angle. It
should be noted, however, that due to the anterior shift of
the tibia in the resting position, the envelope of total a.-p.
translation was shifted anteriorly following the osteotomy.
Thus, a relative increase in ATT occurred, whereas posterior translation decreased. Contrary to the hypothesis of
the authors, no changes in the in situ forces of the ACL
were recorded under any tested loading condition. When
interpreting these results, it is important to note that the
loading conditions applied in this study were much lower
than forces that may occur during activities of daily living
[88, 97]. Given this fact, Fening et al. [24] analyzed knee
kinematics and ACL strain under higher external loads
(209 N a.-p. load, 418 N compressive load) in 5 non-osteotomized cadaveric knees, after a 5 and 10-mm anterior
opening wedge HTO that resulted in increases in tibial
slope of 3.5° and 9.6°, respectively. Following the osteotomy, a significant anterior tibial shift in the resting
position and an increased external rotation of the tibia
relative to the femur were observed. ATT was not significantly affected by the osteotomy. Surprisingly, the
authors found that with increased tibial slope, strain on
the ACL decreased.
The reason for the unexpected strain behaviour of the
ACL after increased tibial slope in the studies by Giffin
et al. [31] and Fening et al. [24] remains unknown. Influential factors might be the increased external tibial rotation,
which may lead to decreased tension of the ACL or a
compensatory increased strain in secondary restraints.
However, Martineau et al. [67], using the same experimental model as Fening et al. [24], did not observe any
significant increases in strain in the medial or lateral collateral ligament after anterior opening wedge HTO.
Another explanation may be the relative superior tibial
translation and anterior shift of the tibiofemoral contact
area observed by Agneskirchner et al. [1]. This shift may
lead to an approximation of the ACL insertion sites despite
a relatively ventralized tibia.
Tibial slope as a risk factor for ACL injury
In a cadaveric study by McLean et al. [70], mean peak
strain in the anteromedial bundle of the ACL was found to
be directly proportional to anterior tibial acceleration during a simulated jump-landing task. More remarkably, the
tibial slope was significantly correlated with both peak
anterior tibial acceleration and peak anteromedial bundle
strain. A steep tibial slope might therefore play a crucial
role in non-contact ACL injuries.
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Several studies have addressed the question of whether a
steep tibial slope on lateral radiographs represents a risk
factor for non-contact ACL injury [11, 45, 71, 94, 99, 104].
Novel insights could be gained from MRI-based studies
[6, 37, 46, 55, 90, 96, 98].
Brandon et al. [11] retrospectively measured the tibial
slope on lateral radiographs (using the proximal tibial anatomical axis) of 100 patients with isolated non-contact ACL
injuries and 100 patients with patellofemoral pain. Subjects
with non-contact ACL injuries showed a significantly steeper tibial slope compared to the control group. This finding
was true for both men and women. Other investigations,
however, found this correlation only in female subjects
[45, 99]. The authors concluded that this observation might
be one reason for the higher incidence of non-contact ACL
injuries seen in women [33]. Using the mechanical axis of
the tibia, Sonnery-Cottet et al. [94] found a statistically
increased tibial slope in 50 patients (35 men, 15 women)
with an isolated rupture of the ACL compared to a control
group matched for age and gender. However, the authors did
not examine each gender independently. In contradiction to
these studies, Meister et al. [71] did not find a significant
difference in tibial slope between 49 patients with noncontact ACL injuries and an age-matched control group of
39 patients with patellofemoral pain syndrome.
Using MRI, Stijak et al. [96] compared the values of the
medial and lateral tibial slope of 33 patients with isolated
ACL injuries and a control group of 33 matched patients
with patellofemoral pain. They found that the lateral tibial
slope was significantly steeper in the ACL-injured group,
whereas the medial tibial slope showed no significant difference. Additionally, a significantly greater difference
between the lateral and medial slope was observed in the
ACL-injured group. In a study by Hashemi et al. [37], a
significantly steeper lateral tibial slope was found in both
male and female subjects with ACL injuries compared to
uninjured controls, whereas a steeper medial tibial slope
was only seen in male subjects. Other MRI-based studies
have confirmed the relationship between a steep lateral
tibial slope and ACL injury [6, 55, 90]. This mechanism
may be explained as follows: Under axial loading, the
lateral femoral condyle slides posteriorly along the lateral
tibial plateau, resulting in a relative external rotation of the
femur or relative internal rotation of the tibia [69, 90]
(Fig. 4). Since external rotation of the femur causes
increased strain on the ACL [25, 66], a steep lateral tibial
slope may contribute to ACL injury. The sliding mechanism of the lateral femoral condyle might be enhanced by
the convex shape of the lateral tibial plateau, which provides less bony stability compared to the concave medial
tibial plateau [3, 7]. Furthermore, the lateral meniscus is
more mobile compared to the medial meniscus thereby
allowing more movement of the lateral condyle [3].
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Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
Fig. 4 Mechanism between a steep lateral tibial slope (green plate)
and increased external rotation of the femur. a Before axial loading
(resting position). b Under axial loading, the lateral femoral condyle
slides posteriorly along the lateral tibial plateau, resulting in an
external rotation of the femur (red arrow)
In contrast, Hudek et al. [46] did not find any significant
differences in the medial or lateral tibial slope between 55
subjects with non-contact ACL injuries and 55 matched
controls with patellofemoral pain syndrome. Remarkably,
ACL-injured subjects in this study showed a significantly
steeper lateral meniscal slope compared to the control group.
This study is the only one to investigate the impact of the
meniscal slope on non-contact ACL injuries so far. This
parameter might be of particular interest for future studies
because the menisci are important contributors to sagittal
and rotational stability [61, 62, 75]. Another interesting
finding observed in the case–control study by Hashemi et al.
[37] was that ACL-injured subjects showed a significantly
lower depth of the concavity of the medial tibial plateau
compared to uninjured controls, regardless of sex. Khan et al.
[55] confirmed this finding in female patients. A shallow
medial plateau may be associated with decreased resistance
to displacement of the tibia relative to the femur because of
less joint congruity, thereby reinforcing the impact of the
bony tibial slope or meniscal slope.
tendon-bone grafts. They investigated the effects of the tibial
slope as measured on lateral radiographs using the posterior
tibial cortex on knee functionality as measured by using the
Cincinnati Knee Rating System. The tibial slope averaged
7.2° and showed no significant correlation with postoperative
knee functionality. However, when the authors divided the
tibial slope into intervals (0–4°, 5–9° and[10°), a significant
correlation was seen: Patients with a steeper tibial slope
showed better functional values. The authors postulated that
an increase in tibial slope might lengthen the hamstring
muscles and enable them to operate over a more efficient
portion of their length-tension relationship, thus enabling
greater control of ATT. Furthermore, the authors argued that
tighter hamstring muscles may enable afferent receptors in the
muscles, tendons and capsule to initiate a more effective
compensatory reflex response. However, the extent to which
hamstring muscle function is influenced by the tibial slope is
not yet completely understood, as further discussed in the
following section. In addition, the authors did not distinguish
between the medial and lateral tibial slope.
Influence of the tibial slope on the outcome
after ACL reconstruction
Influence of the tibial slope on hamstring muscle
function
To date, only one study has examined the relationship
between the tibial slope and outcome after ACL reconstruction. Hohmann et al. [44] looked at 24 patients between 18 and
24 months after ACL reconstruction with bone-patellar-
As previously mentioned, the tibial slope is believed to
influence the function of the hamstring muscles [44, 57, 64].
Because of the resulting anterior shift of the tibia, a steep
tibial slope might lead to increased passive muscle tension
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and optimization of the length-tension relationship in the
hamstring muscles. As a consequence, a steep tibial slope
may theoretically improve their function as passive and
dynamic stabilizers of the knee joint [73, 83, 84]. Since
pathologically increased ATT in the ACL-deficient knee
can be reduced by contraction of the hamstring muscles
[93, 106], a steeper tibial slope might therefore be beneficial to cope with ACL insufficiency. In the study by
Hohmann et al. [44], knee function increased with higher
posterior slope intervals in ACL-deficient patients, confirming this hypothesis.
Kostogiannis et al. [57] studied the effect of the tibial
slope on the need for ACL reconstruction in 100 patients
with complete ACL ruptures who were followed prospectively with the intention of conservative treatment. After
15 years, 22 of 94 available patients had undergone ACL
reconstruction. The mean tibial slope showed no significant
difference between reconstructed and non-reconstructed
knees. However, when patients were divided on the basis
of the tibial slope into four categories, reconstructed knees
were significantly overrepresented among those with
extremely flat tibial slope angles (\7.6°). A flat tibial slope
increased the odds ratio of the need for reconstruction by
an almost fourfold factor. One explanation for this finding
might be that a flat tibial slope potentially decreases the
effectiveness of the hamstring muscles in compensating for
ACL deficiency. In contrast, Liu and Maitland [64] demonstrated in a mathematical model that the ability of the
hamstring muscles to compensate for ATT due to ACL
deficiency during walking was adversely affected by the
tibial slope. In an ACL-deficient knee with a tibial slope of
4°, only 24% of the maximal hamstring muscle force was
required to completely restore the tibia to its normal
position. The required muscle force increased to 66% when
the tibial slope was 8°. In an ACL-deficient knee with 12°
of tibial slope, anterior displacement of the tibia could not
be completely compensated by hamstring muscle force.
The role of slope-modifying HTO in ACL
insufficiency and ACL reconstruction
Valgus HTO is an established treatment option for isolated
medial osteoarthritis and varus malalignment in the knee of
young and active patients [17, 39, 76, 95]. The goal of this
procedure is to change the mechanical weight-bearing axis
by correcting tibial alignment in the frontal plane. However, it has been demonstrated that medial opening wedge
HTO can cause an unintended increase in tibial slope,
whereas lateral closing wedge HTO can cause an unintended decrease in tibial slope [13, 23, 43, 53]. Therefore,
technical modifications have been described to prevent
undesired changes in tibial slope during isolated valgus
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procedures [38, 40, 82] and thereby avoid negative consequences on knee biomechanics [52]. These modifications, however, can also be used to specifically change the
tibial slope during valgus HTO. Furthermore, HTO can be
performed as an isolated flexion or extension procedure
without changing coronal alignment [9, 18, 78].
Slope-increasing sagittal or combined (sagittal and
coronal) HTO is an accepted therapeutic option in the case
of posterior and posterolateral instability combined with
hyperextension and/or varus deformity. This procedure has
been used in isolation or combined with ligamentous
reconstruction [2, 3, 29, 30, 77, 79, 85]. In contrast, isolated
or combined slope-modifying HTO has not been established
as a therapeutic option for ACL insufficiency. Numerous
authors have published the results after combined HTO and
ACL reconstruction [10, 20, 48, 59, 60, 80, 81]. In these
studies, however, HTO was performed with the intention of
correcting varus malalignment in the coronal plane in order
to prevent excessive strain on the graft and progression of
medial compartment osteoarthrosis. The role of the tibial
slope in these procedures has received only little attention.
Nevertheless, a remarkable observation was made in a study
by Dejour et al. [20]. In their follow-up examination of 39
patients after combined HTO and ACL reconstruction,
postoperative ATT correlated with changes of the tibial
slope: ATT was less when the tibial slope was decreased.
The same observation was made by Lerat et al. [60]. The
authors therefore recommend decreasing tibial slope during
combined valgus HTO and ACL reconstruction to augment
ligamentous reconstruction and to prevent mechanical
overloading of the graft.
In a study by Lattermann et al. [59], isolated HTO was
found to be a successful treatment method for a certain
group of patients with ACL insufficiency. The authors
studied the outcome of patients with medial osteoarthritis
and chronic anterior instability using three different treatment modalities: HTO alone, HTO with simultaneous ACL
reconstruction and HTO with secondary ACL reconstruction 6–12 months later. HTO alone was performed in older
patients (38–48 years) whose major complaint was pain
during light daily activity. Subjective instability was also
reported by the patients, but it was not their major complaint. In most of these patients, both pain and symptoms of
instability were reduced with isolated HTO. However, the
authors did not report how much the tibial slope was
altered by the HTO. Nevertheless, these results indicate
that a combined valgus and extension osteotomy might be
appropriate in patients with progressed medial osteoarthritis and low subjective instability.
To date, it is unclear whether an isolated extension
osteotomy without ligament reconstruction can be used as a
therapeutic option in patients with ACL insufficiency. In a
recent cadaveric study, Voos et al. [103] evaluated the
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effect of the tibial slope on laxity of the ACL-deficient
knee. Instrumented Lachman and pivot shift tests were
performed with the ACL intact, after sectioning the ACL
and after a slope-modifying osteotomy that either increased
or decreased tibial slope by 5°. Altering the tibial slope did
not affect ATT during the Lachman test. Interestingly,
however, a 5° increase in tibial slope resulted in a significant increase in ATT during the pivot shift test, while a 5°
decrease in tibial slope reduced ATT during the pivot shift
test to a level similar to that of the intact knee. These
findings correlate with the observation by Brandon et al.
[11], who also found an association between increased
posterior tibial slope and higher pivot shift grades in
patients with ACL insufficiency. On basis of their findings,
Voos et al. [103] concluded that a levelling HTO may
confer a more protective environment to the reconstructed
ACL graft in cases of increased native slope.
Discussion
The tibial slope is believed to influence sagittal plane laxity
and thereby affect the loading behaviour of the ACL.
A steep tibial slope might have adverse impacts on the
ACL-intact, ACL-insufficient and ACL-reconstructed knee
joint. A better understanding of the significance of the
tibial slope may help to prevent ACL injuries and to
improve treatment strategies for ACL insufficiency.
Cadaveric studies have shown that an artificially
increased tibial slope results in an increased anterior shift of
the tibia relative to the femur [1, 24, 31, 67]. While mathematical models additionally have demonstrated an increased
strain in the ACL with increasing tibial slope [86, 87], these
findings have not been confirmed in the aforementioned
cadaveric studies. These data, however, must be interpreted
with caution, as a slope-increasing osteotomy in a cadaveric
knee may increase the loading of secondary restraints and
thereby reduce stress on the ACL [94]. Furthermore, the
effect of additional muscle forces was not studied and the
quadriceps force has a significant impact on the loading
condition of the ACL [22, 72, 105]. Hence, these cadaveric
models may not reflect the real impact of a steep tibial slope
on the strain behaviour of the ACL in a native knee joint [94].
There is some evidence that a steep tibial slope on lateral
radiographs represents a risk factor for non-contact ACL
injury [11, 94]. MRI-based studies indicate that a steep lateral
tibial slope might be particularly responsible for this injury
mechanism [37, 69, 90, 96]. However, some authors found
this correlation to be sex-dependent [6, 45, 55, 98, 99] or
found no correlation at all [46, 71]. Incongruity between these
studies might exist because the aetiology of non-contact ACL
injuries is most likely multifactorial, and other anatomical
variations, such as a narrow intercondylar notch, smaller
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ACL volume and steep lateral meniscal slope, have been
reported to increase the risk of injury [8, 32, 33, 46, 94, 101].
Therefore, it seems difficult to investigate one risk factor
independently, and future research should consider these
anatomical variations in concert [94]. Furthermore, the
mechanical impact of the tibial slope has been reported to be
dependent on the position of the lower limb during landing
[7], indicating that the contribution of the tibial slope to ACL
injury is different for various injury mechanisms. To date, no
uniform definition exists for non-contact ACL injuries,
making patient selection difficult. Thus, inconsistent results
might also be a consequence of different patient selection and
inclusion patterns.
The ability to cope with ACL insufficiency might be
partly related to the tibial slope [27, 57]. A steeper tibial
slope might correlate with greater instability, and therefore,
the tibial slope could serve as a measurable parameter to
identify patients who will most likely not be able to cope
with ACL insufficiency. With respect to the current literature, however, this consideration remains controversial.
Whereas some studies found an association between a
steep tibial slope and greater amount of ATT [19] as well
as higher pivot shift grades [11, 103] in ACL-deficient
knees, other investigations could not confirm these findings
[27, 44, 103].
A steep tibial slope might be a contributing factor for
recurrent instability after ACL reconstruction due to
repetitive overloading and subsequent elongation of the
graft during accelerated rehabilitation with early weightbearing. Therefore, slower rehabilitation with partial
weightbearing for a longer period of time might be beneficial in such patients. On the other hand, a slopedecreasing osteotomy might prevent graft failure over time
in knees with a steep tibial slope. Neyret et al. [78] recommend performing an additional extension osteotomy to
protect the graft when tibial slope exceeds 13°. To date,
however, no evidence exists to confirm this approach. In
our opinion, a slope-decreasing osteotomy should be considered in ACL revision cases, especially in patients with
multiple failed ACL reconstructions.
A slope-decreasing osteotomy might serve as a therapeutic option to treat ACL insufficiency [31, 42]. This
approach is already established in veterinary medicine where
slope-decreasing osteotomies are successfully used to treat
cranial cruciate ligament injuries in dogs [56, 91, 92].
However, only limited data are available to confirm this
treatment option in humans [103]. In our opinion, a slopemodifying HTO without ACL reconstruction might be
reasonable in patients with ACL insufficiency and progressed medial osteoarthritis, but low subjective instability
and low patient activity. In those patients, additional ACL
reconstruction may lead to progression of pain because of
increased tibiofemoral contact pressure [48]. Therefore,
Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
valgus and slope-decreasing HTO without ACL reconstruction might be sufficient in these cases. A differentiated
treatment algorithm for combined varus malalignment or
varus osteoarthritis and anteromedial or posterolateral
instability was proposed by our study group [41], as shown
in Table 1. When a slope-modifying osteotomy alone or in
combination with ligament reconstruction is being considered, it must be noted that changing the tibial slope also
alters the ultimate range of motion. In our experience,
when performing a valgus and slope-decreasing HTO in the
case of anteromedial instability, postoperative hyperextension should not exceed 5° [41].
More recently, several authors have emphasized that
characterizing the tibial plateau surface geometry with a
single slope represents only an insufficient approximation of
its three-dimensionality, and the biomechanical impact of
the tibial slope might be more complex [36, 69, 90].
Because of substantial differences between the slope of the
medial and lateral compartment [16, 35, 47, 50, 54, 58, 107],
axial tibiofemoral loading might also result in rotational
movements between the femur and tibia, due to more
pronounced sliding of the distal femur along the steeper
plateau [7, 69, 90]. Reducing the impact of the tibial slope
solely on sagittal plane kinematics might therefore be an
oversimplification of its true influence on knee biomechanics, neglecting its impact on tibiofemoral rotation. In
our opinion, the most likely explanation for many contradictory results of studies discussed within this article is that
the slope of the medial and lateral plateau has commonly
not been evaluated separately and the effect on tibiofemoral rotation has been widely neglected.
By measuring the tibial slope on lateral radiographs,
only the bony configuration of the tibial plateau is taken
into account. As the posterior horns of the menisci are
usually thicker than the anterior horns, the bony tibial slope
is ultimately reduced by the menisci (Fig. 5). Jenny et al.
[50] therefore introduced the term meniscal slope. These
authors demonstrated a mean difference between the bony
tibial slope and the meniscal slope of 6°, with the meniscal
141
slope being almost perpendicular to the proximal tibial axis
[50]. Given the fact that the menisci contribute to sagittal
and rotational laxity [61, 62, 75], the meniscal slope, rather
than the bony tibial slope, might represent the mechanically relevant slope. Another factor that might contribute to
the mechanical significance of the tibial slope is the depth
of the concavity of the medial tibial plateau [36]. A deep
medial plateau covers a greater amount of the medial
femoral condyle, theoretically leading to a coupling
mechanism with increased femoro-tibial stability. Thus, the
impact of the tibial slope might be small in knees with a
deep medial plateau, whereas a shallow medial plateau
might enhance the mechanical significance of the tibial
Fig. 5 Difference between the bony tibial slope (black line) and the
meniscal slope (white line). Due to the relatively increased thickness
of the posterior horn of the meniscus, the mechanically active slope is
reduced compared to the bony tibial slope
Table 1 Treatment algorithm for combined varus malalignment or varus osteoarthritis and anteromedial or posterolateral instability as proposed
by Hinterwimmer et al. [41]
Varus
malalignment
Varus
osteoarthritis
No instability
Anteromedial instability
Posterolateral instability
Optional ligament
reconstruction or valgus
HTO
ACL reconstruction;
PCL/PLC reconstruction;
Valgus HTO
Valgus HTO when varus [5°;
Valgus HTO when varus [2°;
Extension HTO when TS [10° (maximum 5°
hyperextension postoperatively)
Flexion HTO when extension[0° (minimum
0° extension postoperatively)
Valgus and extension HTO (maximum 5°
hyperextension postoperatively);
Valgus and flexion HTO (minimum 0°
extension postoperatively);
Optional ACL reconstruction (single- or twostage procedure)
Optional PCL/PLC reconstruction (two-stage
procedure)
HTO high tibial osteotomy, PCL posterior cruciate ligament, PLC posterolateral corner, TS tibial slope
123
142
slope [36]. To date, however, no biomechanical data about
the effect of the meniscal slope or the depth of the concavity of the medial tibial plateau on ATT or ACL strain
are available to confirm these hypotheses.
Knee Surg Sports Traumatol Arthrosc (2013) 21:134–145
6.
7.
Conclusion
Cadaveric studies have shown that an artificially increased
tibial slope results in an increased anterior shift of the tibia
relative to the femur. While mathematical models also
demonstrated an increased strain in the ACL with
increasing tibial slope, these findings have not been confirmed in biomechanical models. There is some evidence
that a steep tibial slope represents a risk factor for noncontact ACL injury. MRI-based studies indicate that a
steep lateral tibial slope might be particularly responsible
for this injury mechanism. To date, it is not clear whether a
slope-decreasing osteotomy may be a valuable tool to treat
sagittal plane laxity in the ACL-insufficient knee. Furthermore, it is not known whether a slope-decreasing
osteotomy is necessary in subjects with a steep tibial slope
undergoing ACL reconstruction in order to prevent the
graft. Characterizing the tibial plateau surface geometry
with a single slope on lateral radiographs represents only
an insufficient approximation of its three-dimensionality.
Because of substantial differences between the slope of the
medial and lateral compartment, axial tibiofemoral compression might not only result in ATT, but also in rotational
movements between tibia and femur. Additionally, the
mechanical relevance of the tibial slope might be influenced by other surface parameters such as the meniscal
slope and the depth of the concavity of the medial tibial
plateau.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
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