Relationship of the Intercondylar Roof
and the Tibial Footprint of the ACL
Implications for ACL Reconstruction
Peter T. Scheffel,*yMD, Heath B. Henninger,yzPhD, and Robert T. Burks,yMD
Investigation performed at the University of Utah, Salt Lake City, Utah
Background: Debate exists on the proper relation of the anterior cruciate ligament (ACL) footprint with the intercondylar notch in
anatomic ACL reconstructions. Patient-specific graft placement based on the inclination of the intercondylar roof has been pro-
posed. The relationship between the intercondylar roof and native ACL footprint on the tibia has not previously been quantified.
Hypothesis: No statistical relationship exists between the intercondylar roof angle and the location of the native footprint of the
ACL on the tibia.
Study Design: Case series; Level of evidence, 4.
Methods: Knees from 138 patients with both lateral radiographs and MRI, without a history of ligamentous injury or fracture, were
reviewed to measure the intercondylar roof angle of the femur. Roof angles were measured on lateral radiographs. The MRI data
of the same knees were analyzed to measure the position of the central tibial footprint of the ACL (cACL). The roof angle and tibial
footprint were evaluated to determine if statistical relationships existed.
Results: Patients had a mean 6 SD age of 40 6 16 years. Average roof angle was 34.7? 6 5.2? (range, 23?-48?; 95% CI, 33.9?-
35.5?), and it differed by sex but not by side (right/left). The cACL was 44.1% 6 3.4% (range, 36.1%-51.9%; 95% CI, 43.2%-
45.0%) of the anteroposterior length of the tibia. There was only a weak correlation between the intercondylar roof angle and
the cACL (R = 0.106). No significant differences arose between subpopulations of sex or side.
Conclusion: The tibial footprint of the ACL is located in a position on the tibia that is consistent and does not vary according to
intercondylar roof angle. The cACL is consistently located between 43.2% and 45.0% of the anteroposterior length of the tibia.
Intercondylar roof–based guidance may not predictably place a tibial tunnel in the native ACL footprint. Use of a generic ACL
footprint to place a tibial tunnel during ACL reconstruction may be reliable in up to 95% of patients.
Keywords: ACL; femoral roof angle; impingement; ACL repair
Failure of an anterior cruciate ligament (ACL) graft after
reconstruction remains a concern. The location of graft
placement, and its relationship to subsequent failure, has
been studied extensively.§Impingement of the graft on
the intercondylar roof is one presumed mode of failure
and is thought to cause loss of extension, pain, knee effu-
Iriuchishima et al17,18and Bedi and Altchek2have sug-
gested that proper placement of the ACL graft should be
within the native ACL footprint to prevent impingement
and have referred to this methodology as the ‘‘anatomic’’
reconstruction technique.10Others recommend placement
based on the intercondylar roof angle to avoid impinge-
ment on the femur.12,13,16By either method, the anatomy
dictates the placement of the graft and suggests a possible
variable placement in ACL reconstruction.
Impingement of the graft on the intercondylar roof is
thought to be caused by placement of the graft too far ante-
rior on the tibial plateau, allowing the graft to contact the
roof during knee extension. Anatomic variation may com-
plicate this condition, obscuring the requirements for con-
sistent placement of the graft to avoid impingement.12-14,16
It is reported that knees with more vertical roof angles,
and/or physiological hyperextension, require the graft to
be placed more posterior on the tibia to prevent impinge-
ment against the roof.12Additionally, it is recommended
that the tibial tunnel be aligned parallel to the intercondy-
lar roof when the knee is in maximum extension. If the
native ACL position is dictated by anatomic barriers such
as the intercondylar roof, then patients with a steep roof
angle should have a native tibial footprint relatively
*Address correspondence to Peter T. Scheffel, MD, Department of
Orthopaedics, University of Utah, 1326 South 1000 East, Salt Lake
City, UT 84105 (e-mail: firstname.lastname@example.org).
yDepartment of Orthopaedics, University of Utah, Salt Lake City, Utah.
zDepartment of Bioengineering, University of Utah, Salt Lake City, Utah.
The authors declared that they have no conflicts of interest in the
authorship and publication of this contribution.
The American Journal of Sports Medicine, Vol. 41, No. 2
? 2012 The Author(s)
§References 6-9, 12, 13, 16, 17, 19, 21.
at UNIV OF UTAH SALT LAKE CITY on August 29, 2013 ajs.sagepub.com Downloaded from
posterior on the tibia. This would prevent contact of the lig-
ament in full extension or even hyperextension. While the
femoral side attachment is relatively well defined,3,28evi-
dence of the relationship between all the anatomic factors
is largely unknown. The assumption would be that an ACL
reconstruction should follow anatomic cues. By this meth-
because maximal knee extension and intercondylar roof
angles vary between patients.12,13
specific measures have not been widely accepted.
Interestingly, the footprint of the ACL on the tibial pla-
teau has been shown to remain relatively consistent between
patients despite age and sex, although roof angles have been
found to vary.4,12,13,26This suggests that the native tibial
footprint does not change relative to the roof angle, or the
degree of knee extension, and roof angle measurements alone
may not be reliable to guide ACL reconstructions.
The purpose of this study was to identify the center of the
tibial footprint, and its variability, in knees with intact lig-
aments (using magnetic resonance imaging [MRI]). Addi-
tionally, intercondylar roof angles were measured in the
same knees (from lateral radiographs), and statistical tests
were undertaken to determine if a relationship existed
between the native ACL footprint position and the intercon-
dylar roof angle in ‘‘ligamentously intact’’ knees.
MATERIALS AND METHODS
This retrospective study was approved by the Institutional
Review Board of the University of Utah (#55828). Imaging
data obtained during the normal course of care were
reviewed from patients who presented with complaints
relating to the knee in 2011. Patients were included if
they met the following criteria: (1) a lateral knee radio-
graph was obtained, and (2) an MRI scan of the same
knee demonstrated the absence of a ligamentous abnor-
mality or fracture. Meniscal and chondral injuries did not
exclude the patient from the study.
Radiographic Technique and Interpretation
Lateral plain radiographs were obtained by placing a 14 3
17–inch cassette next to the knee, centered so that the dis-
tal femur and proximal tibia were captured. The beam was
directed medial to lateral, with the appropriate amount of
rotation to allow for condylar overlap. Any image demon-
strating a fracture or .6 mm of condylar offset was
excluded from the study. This exclusion criterion was con-
sistent with previous studies performed to evaluate inter-
condylar roof angles.13,16
Per the technique of Howell and Barad,13the intercon-
dylar roof angle was measured. All image-based measure-
ments were performed by the lead author (P.T.S.) with
tools available in a picture archiving and communication
system (PACS) (iSite PACS v3.6, Philips Healthcare, And-
over, Massachusetts). The intercondylar roof angle was
measured between 2 lines, 1 parallel to the posterior cortex
of the femur and 1 parallel to the intercondylar roof
(Figure 1A). In a previous study, Howell and Barad13
showed the roof angle to be independent of knee extension;
therefore, criteria for specific knee extension angles were
not enforced as exclusion criteria.
MRI Technique and Interpretation
All MRI series were performed with a Siemens Avanto
1.5-T superconducting magnet (Siemens AG, Erlangen,
Germany). Imaging was performed from medial to lateral,
and sagittal imaging produced scans consisting of 3-mm
slices. Each series was reviewed by the lead author
(P.T.S.) and musculoskeletal radiologists to ensure liga-
mentous continuity and the absence of fractures. Any
patient having a displaced fracture or a complete or partial
ligamentous rupture was excluded.
The tibial insertion of the ACL was determined by eval-
uating the T1- and T2-weighted sequences on the sagittal
projection. Each MRI series was reviewed to determine
Figure 1. Schematic of relevant measures (adapted from
Howell and Barad13). (A) Roof angle (a) is measured between
a line parallel to the posterior cortex of the femur and a line
parallel to the intercondylar roof. (B) Length of the tibia is
defined between the anterior and posterior edges of the tibia
in the magnetic resonance imaging slice where the anterior
cruciate ligament (ACL) is widest. Center of the ACL (cACL)
is the midline tibial insertion with respect to the anterior
Vol. 41, No. 2, 2013Intercondylar Roof and Tibial Footprint of the ACL397
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the sagittal slice that represented the largest ACL tibial
footprint. From this image, the center of the tibial insertion
of the ACL (cACL) was measured as a percentage of the
anteroposterior (AP) length of the tibial plateau (Figure
1B). This technique was consistent with previous stud-
ies13,26but cannot account for variation in the mediolateral
and oblique aspects of the footprint.20
Two observers (P.T.S., H.B.H.) evaluated a subset of 15
radiographs and MRI series to determine the interrater
repeated the data set after 4 weeks to determine intrarater
reliability. Intraclass correlation coefficients (ICCs) were
used to evaluate consistency between observers and within
the same observer.
Kolmogorov-Smirnov test to ensure normality of the sample
population. From the normally distributed population, only
patients above and below 1 standard deviation (SD) of the
mean were selected for further analysis. This produced 2 sub-
populations representing the extremes of the range of inter-
condylar roof angles, highlighting differences between the
fringes of the same population and allowing discrete statisti-
cal analyses to be performed within the same overall
The Pearson correlation coefficient was calculated to
determine if relationships existed between the tibial foot-
print (tibial length, cACL) and roof angle. Strength of cor-
relation was assessed as follows: ?0.3 = weak, if any; 0.3-
0.5 = low; 0.5-0.7 = moderate; 0.7-0.9 = high; ?0.9 = very
Discrete comparisons between subpopulations
(sex, side [right/left], upper/lower SD) were performed
with 2-tailed independent t tests with significance at P ?
.05. All data are presented as mean 6 SD (range).
Fifteen images were examined by each of the observers.
Interobserver correlation coefficients for roof angle meas-
urements exhibited strong agreement5(ICC, 0.714; 95%
confidence interval [CI], 0.148-0.904) as well as strong
agreement for cACL (ICC, 0.782; 95% CI, 0.350-0.927).
Intrarater reliability was excellent for roof angle (ICC,
0.958; 95% CI, 0.879-0.986) and strong for cACL (ICC,
0.794; 95% CI, 0.387-0.931). Given the high ICCs, measure-
ments of the variables in this study are considered reliable.
The population of 138 patients had a mean age of 40 6
16 years (range, 18-72). The roof angles, by subpopulation,
are found in Table 1. The average intercondylar roof angle
in 138 patients was 34.7? 6 5.2? (range, 23?-48?; 95% CI,
33.9?-35.5?). The roof angle differed between male and
female patients (P = .005), but no differences were detected
by side (right/left, P = .890). By subpopulation (eg, female,
right), no differences were detected within sex and
between sides (both P ? .579). Between sexes, within
a side, no differences were detected on the right (P =
.099), but differences were detected on the left (P = .020).
The roof angles for all populations (all patients, sex,
side, etc) were normally distributed (all P ? .143). Accord-
ingly, subpopulations with a roof angle ?30? and ?40? (61
SD of the mean) were isolated for further analysis. The
demographics of these 55 patients were as follows: 40 6
17 years (range, 18-72), 26 female, 29 male, 26 right, 29
left. The cACL was reliably located at a position 44.1% 6
3.4% of the AP length of the tibia (95% CI, 43.2%-45.0%)
(Figure 2B), as referenced from the anterior edge of the
For all patients, there was a low correlation between
tibial AP length and roof angle (R = 0.307). When compar-
ing the upper and lower SD populations, significant differ-
ences arose between male and female patients (all P ?
.012) (Figure 2A). No significant differences were found
between upper and lower SD populations within a sex,
within a side, or between sides (all P ? .061).
For all patients, there was a weak correlation between
cACL and roof angle (R = 0.106) (Figure 2B). By subpop-
ulation, female and male patients also showed a weak cor-
relation (R = 0.075 and 0.215, respectively). When
comparing the upper and lower SD populations, no signif-
icant differences arose between sexes or sides, within or
between the upper and lower SD groups (all P ? .097)
Summary of Findings
Analyses showed that in outlier populations of roof angle,
those above and below 1 SD of the mean, the cACL was
reliably located at a position 44.1% 6 3.4% of the AP length
of the tibia (95% CI, 43.2%-45.0%) (Figure 2B), as refer-
enced from the anterior edge of the tibia. Additionally,
the cACL is independent of roof angle. These findings
were not affected by sex or side, but male patients gener-
ally had a longer tibial plateau than female patients
Comparison to Previous Studies
Staubli and Rauschning26measured the intercondylar roof
angle on MRI scans and cryosections of the extended knee.
Descriptive Statistics of Roof Angle by Subpopulationa
Group Roof Angle, deg
All (N = 138)
Female (n = 68)
Male (n = 70)
Right (n = 67)
Left (n = 71)
34.7 6 5.2 (23-48)—
33.4 6 5.1 (24-47)
35.8 6 5.1 (23-48)
34.6 6 5.1 (23-47)
34.8 6 5.4 (24-48)
aData are shown as mean 6 standard deviation (range).
bSignificant difference (P ? .05).
398Scheffel et al The American Journal of Sports Medicine
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The average roof angle from cryosections was 39.8? (range,
35?-44?), which varies considerably from the present study
(34.7? 6 5.2?; range, 23?-48?) given the relatively small var-
iance of the populations. This may be attributed to differ-
ences in data collection between histological sections and
radiographs. While multiple studies have quantified roof
angle to be in the range of 35? to 38?,4,13,23,25,26using var-
ied techniques, these are only slightly lower than the
measurements presented herein. The low variance in the
present study, 65?, is in agreement with previous studies,
affirming that the present findings are reliable and within
the error of prior studies.
Previous studies did not detect significant differences in
roof angle by sex,4,13whereas the present study detected
that female patients (33.4? 6 5.1?) had a lower intercondy-
lar roof angle than male patients (35.8? 6 5.1?) (P = .005)
(Table 1). These differences may not have been detected pre-
viously because of smaller sample sizes. A narrow notch, as
present in female patients, has been identified as a risk fac-
tor for the failure of ACL reconstructions27but was not
related to roof angle and therefore was not within the scope
of the present study. No differences were detected across the
whole population between right and left knees. Expectedly,
the length of the tibial plateau did show significant sex dif-
ferences (Figure 2A) in which male patients had a larger tib-
ial plateau than female patients.
Recurvatum, in the setting of a more vertical intercon-
dylar roof, has been postulated to result in an ‘‘unforgiving
knee’’ where the steep angle creates impingement and
heightens the risk for ACL graft ruptures. Howell and
Barad13found considerable variability in maximal knee
extension between patients, ranging from 2? of flexion to
30? of hyperextension. They determined that the degree
of knee extension was not a reliable indicator of roof angle
and concluded that ‘‘a knee with a given degree of knee
extension was almost as likely to have a vertical roof as
a horizontal roof.’’ Accordingly, the present study did not
include knee extension angle as an independent variable.
Factors like screw home and laxity were not controlled
for in the present study but could also contribute to prema-
ture graft failure. These factors likely do not influence the
anatomic relationship of the intercondylar roof and the tib-
Another potential indicator of impingement may be the
positioning of the ACL graft on the tibial plateau. Howell
and Taylor16described the central position of an unim-
pinged ACL graft as 42% 6 2.8% of the AP dimension of
the tibial plateau, as referenced to the anterior edge. Early
measures of the central ACL footprint were 23 6 4 mm
from the most anterior aspect of the tibia.24More recently,
the native ACL footprint has been measured as having
a mean of 45.6% 6 6.5% in one study and 44% of the AP
length in another,23,26where the anteromedial bundle
and posterolateral bundle of the ACL insert at 41% 6 3%
and 52% 6 3%, respectively.20In the present study, the
cACL was reliably located at a position 44.1% 6 3.4%
(range, 36.1%-51.9%; 95% CI, 43.2%-45.0%) (Figure 2B)
along the length of the tibial plateau, as referenced from
the anterior edge of the tibia. Utilizing this generic position
for placement of a tibial tunnel might result in an
unimpinged ACL graft per the definition of Howell and
Taylor16despite not basing the tunnel location off of the
respective intercondylar roof angle. A recent study by
Mall et al22noted that ‘‘only modest agreement’’ exists
Female (R) Female (L) Male (R)Male (L)
Tibial A-P Length (mm)
Roof Angle (deg)
202530 3540 4550
Female (R) Female (L)Male (R) Male (L)
Figure 2. Tibial length and center of the tibial insertion of the
anterior cruciate ligament (cACL) for upper and lower stan-
dard deviation populations. (A) The anteroposterior length
of the tibia was shorter for female patients (*all P ? .012);
otherwise, there were no differences between or within
groups (all P ? .061). (B) The cACL had low correlation to
roof angle (R = 0.106). (C) No statistical differences for
cACL were detected between or within groups (all P ? .097).
Vol. 41, No. 2, 2013 Intercondylar Roof and Tibial Footprint of the ACL 399
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between numerous studies defining optimal tunnel place-
ment. Their study highlighted how critical the tibial tunnel
was in successful ACL graft placement, suggesting it may
be more important than femoral tunnel placement in pro-
viding stability to the reconstructed knee.
Relationship of ACL Footprint
and Intercondylar Roof Angle
The present study shows that the location of the cACL,
44.1% 6 3.4% of the AP length of the tibia, has a weak rela-
tionship to the intercondylar roof angle (a = 34.7? 6 5.2?;
R = 0.106) (Figure 2B). These findings show that roof angle
is independent of cACL location, and this does not change
by sex or side (Figure 2C). Therefore, intercondylar roof
guidance may be a less accurate method to predictably
place a tibial tunnel within the native ACL footprint.
This conclusion is strengthened by the use of the ‘‘outlier’’
populations, those above and below 1 SD of the mean,
because even these disparate data points cannot detect
any statistical differences. Use of a generic ACL tibial tun-
nel position during ACL reconstructions may be reliable in
up to 95% of patients. However, in a population of 138
patients, the footprint was almost 16%.
The implications of these findings are as follows: (1) The
location of the native ACL footprint is independent of roof
angle, sex, side, or knee extension. (2) Placement of the tib-
ial tunnel should target roughly 44% of the distance along
the tibial plateau, as referenced to the anterior edge of the
tibia as measured with MRI. (3) The accuracy of using inter-
condylar roof guidance to locate the native tibial footprint
during ACL reconstruction remains unknown.
Foremost, this study defined the relationship between fem-
oral roof angle and tibial footprint in otherwise healthy
knees. This study assumed that normal ACLs do not
impinge and cannot conclude the source or mechanism of
impingement in native or grafted ACLs if and when it
exists. Factors like graft shape and diameter are likely crit-
ical in defining impingement. Additionally, all measures of
cACL were referenced to the anterior edge of the tibia as
seen in MRI. Accordingly, this relative definition of the
ACL placement is required in cases where the stump of
a ruptured ACL is not present during reconstruction. Fur-
ther research is required to reliably define these criteria in
the operating theater. Second, radiographic measures may
be confounded by rotation of the extremity, but use of the
‘‘6-mm rule’’ ensured that all measurements were taken
on radiographs that met standard requirements for preop-
erative planning. Similarly, sagittal plane scans from MRI
may not perfectly section the tibial plateau such that the
maximal length is visualized. Given the small SDs in
length and cACL (\5%), these measures, along with the
population size, are considered sufficient to detect any sta-
tistical relationships. Finally, correlation of bony land-
marks between radiographs and MRI has yet to be
defined. As a result, MRI-based measurements of the AP
length through the widest section of the ACL cannot neces-
sarily be directly translated to radiographs.
The intercondylar roof angle likely has no clinical relation-
ship to the location of the central ACL footprint in knees
without a bony deformity or ligament rupture. Use of
a generic ACL footprint position (eg, with intraoperative
fluoroscopy measurements) for a tibial tunnel during
ACL reconstructions may be reliable in up to 95% of
patients, although in a population of 138 patients, the foot-
print was almost 16% of the AP dimension of the tibia.
Therefore, maintaining the footprint and using it to accu-
rately position the tibial tunnel appear reliable.
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