Content uploaded by Teppo L N Järvinen
Author content
All content in this area was uploaded by Teppo L N Järvinen
Content may be subject to copyright.
Initial Fixation Strength of Bioabsorbable
and Titanium Interference Screws in
Anterior Cruciate Ligament Reconstruction
Biomechanical Evaluation by Single Cycle and Cyclic
Loading
Petteri Kousa,*† MD, Teppo L. N. Ja¨rvinen,*† MD, PhD, Pekka Kannus,‡ MD, PhD, and
Markku Ja¨rvinen,*†§ MD, PhD
From the *Medical School and Institute of Medical Technology, University of Tampere, the
†Department of Surgery, Tampere University Hospital, and the ‡Accident and Trauma
Research Center and Tampere Research Center of Sports Medicine, Urho Kaleva Kekkonen
Institute for Health Promotion Research, Tampere, Finland
ABSTRACT
We evaluated the initial bone-patellar tendon-bone graft
fixation strength of bioabsorbable as compared with tita-
nium interference screws in anterior cruciate ligament
reconstruction using matched pairs of porcine knees. Ten
pairs underwent single-cycle failure loading at a rate of 50
mm/min, and 10 pairs underwent cyclic loading at half-
hertz frequency. The cyclic loading started with 100 load
cycles between 50 and 150 N. We then progressively
increased loads in 50-N increments after each set of 100
cycles. After 100 cycles at 850 N, the specimens were
loaded to failure at a rate of 50 mm/min. In the single-
cycle failure loading test, the mean ultimate failure loads
(⫾SD) for the bioabsorbable (837 ⫾ 260 N) and titanium
interference screws (863 ⫾ 192 N) were not significantly
different, norwere the mean yield loads or the stiffness of
the fixation. In the cyclic loading test, the yield loads were
605 ⫾ 142 N and 585 ⫾ 103 N for the bioabsorbable and
titanium interference screws, respectively (no significant
difference). Although there was no significant difference
in the ultimate failure load, more bone block fractures
were found in the grafts fixed with a titanium interference
screw. Bioabsorbable interference screw fixation thus
seems to provide a reasonable alternative to titanium
screws.
An ACL rupture is a common consequence of a knee in-
jury, leading to abnormal kinematics of the knee, which
may, in turn, result in damage to the menisci and articu-
lar cartilage and ultimately lead to osteoarthrosis of the
joint.
20,32,50
Surgical reconstruction of the ACL aims to
reestablish the normal function of the knee and thus pre-
vent the progression of intraarticular damage. The bone-
patellar tendon-bone autograft is the most commonly used
graft in ACL reconstruction.
13,19
It has been suggested that
a bone-patellar tendon-bone graft may lose its initial
strength during healing,
12,15,30
although during the imme-
diate postoperative period the fixation of the graft is the
primary factor in limiting early aggressive rehabilitation.
22
The interference technique introduced by Lambert
23
has been shown to provide better initial fixation strength
than other methods of bone-patellar tendon-bone graft
fixation.
22,33,48
Consequently, an interference fixation
technique using metal interference screws has become the
standard for bone-patellar tendon-bone grafts in ACL sur-
gery.
13,19
The preliminary clinical studies have suggested
that bioabsorbable interference screws provide a reason-
able alternative to metal interference screws in the fixa-
tion of bone-patellar tendon-bone grafts.
5,27,46
In general,
both screws have provided comparable initial fixation
strengths in biomechanical tests,
1,10,17,21,41,43,52
al-
though Pena et al.
34
reported significantly lower fixation
strengths for bioabsorbable interference screws. The
study by Seil et al.
43
is the only study comparing the
response of bioabsorbable and metallic interference
screws to cyclic loading.
Despite the good fixation of bone-patellar tendon-bone
grafts in ACL reconstruction provided by metal interfer-
§ Address correspondence and reprint requests to Markku Ja¨rvinen, MD,
PhD, Department of Surgery, Medical School, University of Tampere, POB
607, 33101 Tampere, Finland.
Three authors have a commercial affiliation with a product named in this
study.
0363-5465/101/2929-0420$02.00/0
THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 29, No. 4
© 2001 American Orthopaedic Society for Sports Medicine
420
ence screws, their bioabsorbable counterparts offer other
obvious advantages, including undistorted MRI, de-
creased risk for graft laceration, decreased stress protec-
tion, no need for hardware removal, and lack of corro-
sion.
4,7,17,42,44,49
These additional benefits have made
the bioabsorbable devices a fascinating option in knee
ligament surgery.
The purpose of this study was to compare the initial
fixation strength of bioabsorbable and standard titanium
interference screws in the bone-patellar tendon-bone re-
construction of the ACL using both single-cycle and cyclic
loading tests. The latter testing was considered especially
important because nowadays ACL patients are treated
postoperatively with early rehabilitation, and thus the
operated knee and its ligaments undergo early repetitive
loading.
6
MATERIALS AND METHODS
Specimen Preparation
Twenty fresh porcine knee pairs (40 knees) were used.
Specimens were obtained from a local slaughterhouse at
the time the animals were killed, fresh-frozen at ⫺20°C,
and thawed for 12 hours at room temperature before test-
ing. The muscles and other soft tissues were removed,
leaving the proximal tibia, patella, and the patellar ten-
don intact. The bone-patellar tendon-bone graft was pre-
pared by obtaining a triangular tibial bone block. A 9-mm
wide dissection of the middle third of the patellar tendon
was then performed, leaving the patella intact. The tibial
bone block was trimmed into a rectangular shape measur-
ing 7 ⫻ 9 ⫻ 20 mm (thickness ⫻ width ⫻ length). To
prepare for the graft fixation into the tibia, a guide wire
was first drilled from the original tibial insertion of the
ACL to the anteromedial wall of the proximal tibia. A
9-mm drill hole was then made using a cannulated drill.
Study Groups
After the preparations were complete, the 20 pairs of knee
specimens were divided into two study groups so that, of
each pair, the ipsilateral and contralateral knees went
into different groups. In the first group (bioabsorbable
group), the 20 bone-patellar tendon-bone grafts were fixed
into the tibia with a 7 ⫻ 25 mm gamma-sterilized bioab-
sorbable self-reinforced
L-lactide/D-lactide (PLA 96/4) co-
polymer interference screw (Bionx Implants Ltd., Tam-
pere, Finland). In the second group (titanium group), a
standard 7 ⫻ 25 mm titanium interference screw (Soft-
silk, Acufex Microsurgical Inc., Mansfield, Massachusetts)
was used. In addition, before insertion of the bioabsorb-
able screw, a 4.5-mm drill and Bionx dilator were used to
create a guide notch to the tunnel opening for screw in-
sertion. All screws were inserted with an outside-in (rear
entry) technique, placing the screw into the 3-mm gap
between the tibial bone tunnel wall and the graft bone
block. For screw insertion, a K-wire was used to prevent
screw-graft divergence.
Biomechanical Testing
The tibia was securely mounted with a specially designed
clamp, and the other end of the system, the patella, was
secured with an 8-mm bar passed through the patellar
bone perpendicular to the long axis of the patella. The
biomechanical testing was then performed using a Lloyd
LR 5K mechanical testing machine (J. J. Lloyd Instru-
ments, Southampton, United Kingdom). The tests were
performed at room temperature, and the specimens were
kept moist with saline spray during preparation and test-
ing. The biomechanical testing protocol included both the
single-cycle load-to-failure test (randomly selected, 10
pairs) and the cyclic loading test (remaining 10 pairs). The
loads were applied parallel to the long axis of the tibial
bone tunnels until fixation failure, bone block fracture of
the graft, or graft tendon disruption.
Single-Cycle Load-to-Failure Test
A vertical tensile loading was performed at a rate of 50
mm/min until failure. The response of the specimen was
automatically obtained in the form of a force-displacement
curve, and the yield load (determined as the point on the
force-displacement curve where the slope first clearly de-
creased) as well as the ultimate failure load, the stiffness,
and the failure mode were determined. Stiffness was de-
termined as the slope of the linear region of the force-
displacement curve.
Cyclic Loading Test
A 50-N preload (the average preload imposed by sur-
geons
2
) was first applied to the specimens, after which the
actual cyclic loading was started at a frequency of one
cycle in 2 seconds (0.5 Hz), beginning with 100 load cycles
to a force level of 150 N (100 load cycles between 50 and
150 N). The load was then progressively increased in
increments of 50 N after each set of 100 cycles. After 100
cycles at 850 N, the surviving specimens were then loaded
to failure at a rate of 50 mm/min. The force-displacement
curve was recorded, and the yield load, determined as the
load at which the displacement first increased signifi-
cantly, as well as the ultimate failure load were deter-
mined (Fig. 1). In addition, the graft displacement at the
first peak of load at each force level was measured to
estimate the resistance of fixation to slipping (Fig. 2).
Similar to the single-cycle load-to-failure testing, the fail-
ure mode (bone block pullout, bone block fracture, or ten-
don rupture) was also analyzed for each specimen.
Statistical Analysis
Differences between the groups were determined using
paired sample t-tests. Decrease in yield load values was
further compared using unpaired t-tests. To investigate
the relationship between screw type and the number of
bone block fractures, a McNemar test was performed. A
value of P less than 0.05 was considered statistically
significant.
Vol. 29, No. 4, 2001 Bioabsorbable versus Metal Screws for ACL Reconstruction 421
RESULTS
Single-Cycle Load-to-Failure Test
The mean yield load was 621 ⫾ 139 N in the bioabsorbable
group and 774 ⫾ 154 N in the titanium group (P ⫽ 0.07).
The mean ultimate failure loads in the two groups were
837 ⫾ 260 N and 863 ⫾ 192N(P ⫽ 0.84), respectively.
Significant difference was not found in the stiffness of the
fixation (76 ⫾ 20 N/mm and 80 ⫾ 15 N/mm, P ⫽ 0.70).
In the failure mode analysis, all specimens in the bio-
absorbable group failed by bone block pullout; in the tita-
nium group, bone block pullout occurred in seven, bone
block fracture in two, and tendon rupture in one specimen.
Significant group differences were not found in the failure
mode of the specimen.
Cyclic Loading Test
The mean yield load was 605 ⫾ 142 N for the bioabsorb-
able screw and 585 ⫾ 103 N for the titanium screw (P ⫽
0.71), and the mean ultimate failure loads were 708 ⫾ 115
N and 683 ⫾ 136N(P ⫽ 0.71), respectively. At none of the
force levels used for cyclic loading was there significant
difference between the groups concerning the graft dis-
placement (Fig. 2).
The failure mode analysis showed that in the bioabsorb-
able group, bone block pullout occurred in eight, bone
block fracture in one, and tendon rupture in one specimen.
In the titanium group, bone block pullout occurred in five,
bone block fracture in four, and tendon rupture in one
specimen. The number of bone block fractures was not
significantly different between the groups (P ⫽ 0.25). The
decrease in yield load values was significant in the tita-
nium group after cyclic loading (P ⫽ 0.03).
DISCUSSION
The fixation of a bone-patellar tendon-bone graft is the
most critical point of an ACL reconstruction in the early
postoperative period until some bone block healing occurs
(between 4 to 12 weeks), after which the graft material
becomes the weakest link of the reconstruction.
12
As
stated by Beynnon and Amis,
6
the single-cycle load-to-
failure test provides a measure of the upper limit of the
graft fixation construct, which is useful information with
regard to the behavior of the graft during unexpected
loading events, such as loss of balance or a fall. During
early intense rehabilitation of the operated knee, however,
the graft fixation construct is subjected to repetitive sub-
maximal loading. Thus, the time-zero ultimate failure
loads do not necessarily adequately describe the behavior
of a graft fixation subjected to cyclic loading.
12
Despite
this, bone-patellar tendon-bone graft fixations have previ-
ously been evaluated almost entirely by single-cycle load-
ing tests.
We used both single-cycle loading and cyclic loading
tests to evaluate the initial strength of two different in-
terference screws for bone-patellar tendon-bone graft fix-
ation. The cyclic loading test allowed us to evaluate the
cyclic response of the interference fixation and to deter-
mine changes (possible deterioration of the screw-graft
interface) that may occur in the initial strength of the
fixation. We also attempted to minimize the specimen-
related variability in our test protocol by using matched
pairs of skeletally mature porcine knees and by loading
the specimens uniaxially (one end of the graft was fixed
into a tibial drill hole and the other end was attached to
the testing machine).
6,41,43
Because our cyclic loading protocol to test the integrity
of the ACL reconstruct was not the most commonly used
one, that is, in which the ACL reconstruct is loaded at very
low constant load levels over multiple cycles, an explana-
tion of the rationale of the method of cyclic loading is
warranted. Actually, we are not the first to use a cyclic
loading protocol in which the loading is progressively in-
creased to test the integrity of ACL graft fixation.
24,25,45
Figure 1. A typical force-displacement curve of the cyclic
loading test showing the loading levels used and the yield
point at which the displacement first began to increase
markedly.
Figure 2. The mean displacement of the fixation at each
force level up to the average yield point in the cyclic loading
test. Significant displacement differences were not observed
between the grafts fixed with bioabsorbable or titanium inter-
ference screws. The values are given as the mean and
standard deviation.
422 Kousa et al. American Journal of Sports Medicine
Most recently, Giurea et al.
14
used a testing protocol in
which the ACL graft constructs were subjected to two
different cyclic loading levels: very low-level loading (0 to
150 N), simulating walking, and moderate-level loading
(same protocol with a 450-N peak load), simulating jog-
ging. In this study, the appropriateness of the higher
loading regimen was strikingly displayed. An 8-mm soft
screw provided good initial fixation strength (879 ⫾ 74 N)
and survived 1100 loading cycles at low-level loads (0 to
150 N), but all specimens failed very rapidly when loaded
cyclically to 450 N.
The actual forces the graft is subjected to are not clearly
known, but it has been estimated that the graft is loaded
to approximately 150 to 500 N during normal activi-
ties.
16,26,31
Recently, Toutoungi et al.
51
showed that an
isokinetic/isometric extension of the knee produces peak
forces 0.55 times body weight. In addition, Rupp et al.
40
showed that quadriceps muscle pull against gravity alone
produces resultant forces up to 247 N in the ACL. In
summary, the objective of the cyclic loading protocol that
we used was to cover the entire range of loading that
might possibly be experienced by a graft during normal
human locomotion. Multiple cycles were applied at very
low-level loads (50 to 250 N; 300 cycles) to simulate walk-
ing, at moderate-level loads (250 to 500 N; 500 cycles) to
simulate jogging, and also at high-level loads (550 to 850
N; 600 cycles) to simulate strenuous activities. Although
this protocol does not use the most common cyclic loading
method, we think that it provides a valid method to com-
pare the two different interference screws.
It is generally agreed that during early aggressive re-
habilitation, the ACL graft construct is subjected to thou-
sands of loading cycles and, thus, the time-zero ultimate
failure loads do not appropriately reflect the possible
changes that occur in fixation strength under cyclic load-
ing at that time.
12
Thus, we performed the single-cycle
(pull-out) test both alone (without prior cyclic loading, 10
knee pairs) and after the cyclic loading test (for those
surviving cyclic loading) to be able to determine the pos-
sible change (deterioration) of the strength of the fixation
during cyclic loading.
The displacement seen in cyclic loading could be due to
either the creep (viscoelastic properties) of the tendon, the
migration of the bone block in the tunnel, or the failure of
the specimen fixation. There were similar displacement
values at each loading level between the two screws, and
the testing protocol was carefully standardized between
the study groups, with knee pairs randomly allocated to
either the metal or bioabsorbable group and the grafts
trimmed to identical size and shape. Thus, we strongly
suspect that both fixations failed to the same extent. The
tendon creep can be assumed to be similar at each force
level.
Both the bioabsorbable and conventional titanium
screws provided similar fixation strengths in the single-
cycle and cyclic loading tests. Our single-cycle loading test
results of the titanium interference screw were also very
comparable with those of previous studies in which either
young human cadaveric or porcine knee specimens and
metal interference screws were used for bone-patellar ten-
don-bone graft fixation.
8,9,28,33–35,37,38,41,47
Although
Pena et al.
34
reported significantly lower fixation strength
using bioabsorbable rather than standard metal interfer-
ence screws, bioabsorbable and metal interference screws
have generally yielded comparable initial fixation
strengths in single-cycle load-to-failure tests.
1,10,18,41,51
In a comprehensive evaluation of the fixation strengths of
six different bioabsorbable screws and a conventional ti-
tanium interference screw, Weiler et al.
52
reported that
five of the tested bioabsorbable screws (one of which was
identical to that used by Pena et al.) provided comparable
fixation strength to that of the titanium screw. The au-
thors suggested that the discrepancy between their re-
sults and those obtained by Pena et al. could be attribut-
able to the differences in the bone quality, the geometry of
the bone block, and the gap size between the screw and the
bone tunnel.
In single-cycle loading studies, the interference fixation
typically fails by bone block pullout, leaving the screw and
part of the cancellous bone of the bone block in the tunnel.
On the basis of reports in the current literature, it seems
that metal interference screws more often cause bone
block fractures compared with bioabsorbable interference
screws.
10,34,41,52
Similarly, we also observed more bone
block fractures in the metal interference screw group as
compared with its bioabsorbable counterpart (two versus
none in the single-cycle test), a phenomenon that seemed
to be emphasized in cyclic loading (four with the titanium
screws versus one with the bioabsorbable screws). How-
ever, the number of bone block fractures was not statisti-
cally different between the groups. A change from single-
cycle loading to cyclic loading resulted in significantly
decreased yield load values in the titanium group (⫺189
N), whereas the yield load values remained practically
unchanged in the bioabsorbable group (⫺16 N). This could
be due to the obvious mismatch of the elastic modulus
between the bone (cancellous bone, 0.2 to 0.7 GPa; cortical
bone, 9 to 20 GPa), the titanium screw (titanium alloy, 110
GPa),
3,29,39
and the bioabsorbable screws (PLA 96/4, 4.6
to 7.5 GPa).
36
Although clinical failure of a metal inter-
ference screw fixation is rare, it has been reported to occur
among patients who have undergone very aggressive re-
habilitation or in those whose compliance with rehabilita-
tion has been poor.
11
To our knowledge, the study by Seil et al.
43
is the only
one comparing biodegradable and metal interference
screws under cyclic loading conditions when bone-patellar
tendon-bone graft has been used. They evaluated the fix-
ation strength of bioabsorbable and metal interference
screws by subjecting porcine graft-fixation constructs to
500 loading cycles between 60 and 250 N at a rate of 300
mm/min. After this cyclic loading, the specimens were
loaded to failure at a rate of 50 mm/min. No statistically
significant difference was found in ultimate failure loads
between the bioabsorbable and titanium interference
screws, and no bone block slippage was observed during
the cyclic loading. However, in contrast to their study, our
testing model covered the whole loading range, starting
from very low-level loads and proceeding to the level of
strenuous activity. Furthermore, in contrast to the results
Vol. 29, No. 4, 2001 Bioabsorbable versus Metal Screws for ACL Reconstruction 423
of our study, they observed more bone block fractures with
bioabsorbable screws than with metal interference screws
in a simple pullout after 500 submaximal loading cycles.
36
We believe that the discrepancy between these two studies
could be attributable to the difference in screw insertion
technique. Seil et al.
43
used a screw tap before the inser-
tion of the bioabsorbable screw, a procedure which may
have damaged the graft and therefore resulted in a higher
occurrence of bone block fractures. Despite a similar tun-
nel-bone block gap, Pena et al.
34
and Weiler et al.
52
re-
ported higher insertion torques for metal than for bioab-
sorbable interference screws.
On the basis of the biomechanical testing results we
obtained with the two interference screws, we think that
the bioabsorbable interference screw provides a reason-
able alternative to the titanium interference screw in fix-
ation of a bone-patellar tendon-bone graft in ACL
reconstruction.
ACKNOWLEDGMENTS
The authors thank Smith & Nephew, Ltd. and Bionx
Implants, Ltd. for providing us with the test implants.
REFERENCES
1. Abate JA,Fadale PD, HulstynMJ, etal: Initialfixation strength ofpolylactic
acid interference screws in anterior cruciate ligament reconstruction
. Ar-
throscopy 14:
278–284, 1998
2. Amis AA, Jakob RP: Anterior cruciate ligament graft positioning, tension-
ing and twisting.
Knee Surg Sports Traumatol Arthrosc 6 (Suppl 1):
S2–S12, 1998
3. Anderson MJ,Keyak JH, SkinnerHB: Compressivemechanical properties
of human cancellous bone after gamma irradiation.
J Bone Joint Surg
74A:
747–752, 1992
4. Bach BR Jr: Potential pitfalls of Kurosaka screw interference fixation for
ACL surgery.
Am J Knee Surg 2:
76–82, 1989
5. Barber FA, Elrod BF, McGuire DA, et al: Preliminary results of an absorb-
able interference screw.
Arthroscopy 11:
537–548, 1995
6. Beynnon BD, Amis AA: In vitro testing protocols for the cruciate ligaments
and ligament reconstructions.
Knee Surg Sports Traumatol Arthrosc 6
(Suppl 1):
S70–S76, 1998
7. Brown CH Jr, Carson EW: Revision anterior cruciate ligament surgery.
Clin Sports Med 18:
109–171, 1999
8. Brown GA, Pen˜a F, Grøntvedt T, et al: Fixation strength of interference
screw fixationin bovine,young human,and elderlyhuman cadaverknees:
Influence of insertion torque, tunnel-bone block gap, and interference.
Knee Surg Sports Traumatol Arthrosc 3:
238–244, 1996
9. Butler JC, Branch TP, Hutton WC: Optimal graft fixation. The effect of gap
size and screw size on bone plug fixation in ACL reconstruction.
Arthros-
copy 10:
524–529, 1994
10. Caborn DNM, Urban WP Jr, Johnson DL, et al: Biomechanical compari-
son between BioScrew and titanium alloy interference screws for bone-
patellar tendon-bonegraft fixationin anteriorcruciate ligamentreconstruc-
tion.
Arthroscopy 13:
229–232, 1997
11. Cooper DE, Wilson TW: Clinical failure of tibial interference screw fixation
after anterior cruciate ligament reconstruction. A report of two cases.
Am J Sports Med 24:
693–697, 1996
12. Corsetti JR, Jackson DW: Failure of anterior cruciate ligament reconstruc-
tion: The biologic basis.
Clin Orthop 325:
42–49, 1996
13. Frank CB, Jackson DW: The science of reconstruction of the anterior
cruciate ligament [currentconcepts review].
J BoneJoint Surg79A:
1556–
1576, 1997
14. Giurea M, Zorilla P, Amis AA, et al: Comparative pull-out and cyclic-
loading strength tests of anchorage of hamstring tendon grafts in anterior
cruciate ligament reconstruction.
Am J Sports Med 27:
621–625, 1999
15. Grana WA, Egle DM, Mahnken R, et al: An analysis of autograft fixation
after anterior cruciate ligament reconstruction in a rabbit model.
Am J Sports Med 22:
344–351, 1994
16. Holden JP, Grood ES, Korvick DL, et al: In vivo forces in the anterior
cruciate ligament: Direct measurements during walking and trotting in a
quadruped.
J Biomech 27:
517–526, 1994
17. Jacobs JJ, Gilbert JL, Urban RM: Corrosion of metal orthopaedic implants
[current concepts review].
J Bone Joint Surg 80A:
268–282, 1998
18. Johnson LL, vanDyk GE: Metal and biodegradable interference screws:
Comparison of failure strength.
Arthroscopy 12:
452–456, 1996
19. Johnson RJ, Beynnon BD, Nichols CE, et al: The treatment of injuries of
the anterior cruciate ligament [current concepts review].
J Bone Joint Surg
74A:
140–151, 1992
20. Kannus P, Ja¨rvinen M: Conservatively treated tears of the anterior cruci-
ate ligament. Long-term results.
J Bone Joint Surg 69A:
1007–1012, 1987
21. Kousa P, Ja¨rvinen TLN, Pohjonen T, et al: Fixation strength of a biode-
gradable screw in anterior cruciate ligament reconstruction.
J Bone Joint
Surg 77B:
901–905, 1995
22. Kurosaka M, Yoshiya S, Andrish JT: A biomechanical comparison of
different surgical techniques of graft fixation in anterior cruciate ligament
reconstruction.
Am J Sports Med 15:
225–229, 1987
23. Lambert KL: Vascularized patellar tendon graft with rigid internal fixation
for anterior cruciate ligament insufficiency.
Clin Orthop 172:
85–89, 1983
24. Liu SH, Kabo JM, Osti L: Biomechanics of two types of bone-tendon-bone
graft for ACL reconstruction.
J Bone Joint Surg 77B:
232–235, 1995
25. Magen HE, Howell SM, Hull ML: Structural properties of six tibial fixation
methods for anterior cruciate ligament soft tissue grafts.
Am J Sports Med
27:
35–43, 1999
26. Markolf KL, Burchfield DM, Shapiro MM, et al: Biomechanical conse-
quences of replacement of the anterior cruciate ligament with a patellar
ligament allograft. Part II: Forces in the graft compared with forces in the
intact ligament.
J Bone Joint Surg 78A:
1728–1734, 1996
27. Marti C, Imhoff AB, Bahrs C, et al: Metallic versus bioabsorbable interfer-
ence screws for fixation of bone-patellar tendon-bone autograft in arthro-
scopic anterior cruciate ligament reconstruction. A preliminary report.
Knee Surg Sports Traumatol Arthrosc 5:
217–221, 1997
28. Matthews LS,Lawrence SJ,Yahiro MA,et al:Fixation strengthsof patellar
tendon-bone grafts.
Arthroscopy 9:
76–81, 1993
29. McCalden RW, McGeough JA, Barker MB, et al: Age-related changes in
the tensile properties of cortical bone. The relative importance of changes
in porosity, mineralization, and microstructure.
J Bone Joint Surg 75A:
1193–1205, 1993
30. McFarland EG, Morrey BF, An KN, et al: The relationship of vascularity
and water content to tensile strength in a patellar tendon replacement of
the anterior cruciate in dogs.
Am J Sports Med 14:
436–448, 1986
31. Noyes FR, Butler DL, Grood ES, et al: Biomechanical analysis of human
ligament grafts used in knee-ligament repairs and reconstructions.
J Bone
Joint Surg 66A:
344–352, 1984
32. Noyes FR, Mooar PA, Matthews DS, et al: The symptomatic anterior
cruciate-deficient knee. Part I. The long-term functional disability in ath-
letically active individuals.
J Bone Joint Surg 65A:
154–162, 1983
33. Paschal SO,Seemann MD,Ashman RB,et al:Interference fixationversus
postfixation of bone-patellar tendon-bone grafts for anterior cruciate liga-
ment reconstruction: A biomechanical comparative study in porcine
knees.
Clin Orthop 300:
281–287, 1994
34. Pena F, Grøntvedt T, Brown GA, et al: Comparison of failure strength
between metallic and absorbable interference screws: Influence of inser-
tion torque, tunnel-bone block gap, bone mineral density, and interfer-
ence.
Am J Sports Med 24:
329–334, 1996
35. Pierz K,Baltz M,Fulkerson J:The effect of Kurosakascrew divergenceon
the holding strength of bone-tendon-bone grafts.
Am J Sports Med 23:
332–335, 1995
36. Pohjonen T, To¨rma¨la¨ P: In vitro hydrolysis of self-reinforced polylactide
composites.
Med Biol Eng Compu 34 (Suppl 1, Part 1):
127–128, 1996
37. Pomeroy G, Baltz M, Pierz K, et al: The effects of bone plug length and
screw diameter on the holding strength of bone-tendon-bone grafts.
Ar-
throscopy 14:
148–152, 1998
38. Reznik AM, Davis JL, Daniel DM: Optimizing interference fixation for
cruciate ligament reconstruction.
Trans Orthop Res Soc 15:
519, 1990
39. Ruluff D, McIntyre PE: Some current uses for metals in, on and around
your body
. Mech Eng 4:
40–47, 1982
40. Rupp S, Hopf T, Hess T, et al: Resulting tensile forces in the human
bone-patellar tendon-bone graft: Direct force measurement in vitro.
Ar-
throscopy 15:
179–184, 1999
41. Rupp S, Krauss PW, Fritsch EW: Fixation strength of a biodegradable
interference screw and a press-fit technique in anterior cruciate ligament
reconstruction with a BPTB graft.
Arthroscopy 13:
61–65, 1997
42. Safran MR, Harner CD: Technical considerations of revision anterior
cruciate ligament surgery.
Clin Orthop 325:
50–64, 1996
43. Seil R, Rupp S, Krauss PW, et al: Comparison of initial fixation strength
between biodegradable and metallic interference screws and a press-fit
fixation technique in a porcine model.
Am J Sports Med 26:
815–819,
1998
424 Kousa et al. American Journal of Sports Medicine
44. Shellock FG, Mink JH, Curtin S, et al: MR imaging and metallic implants
for anterior cruciate ligament reconstruction: Assessment of ferromag-
netism and artifact.
J Magn Reson Imaging 2:
225–228, 1992
45. Stadelmaier DM, Lowe WR, Ilahi OA, et al: Cyclic pull-out strength of
hamstring tendon graft fixation with soft tissue interference screws. Influ-
ence of screw length.
Am J Sports Med 27:
778–783, 1999
46. Sta¨helin AC, Weiler A, Ru¨fenacht H, et al: Clinical degradation and bio-
compatibility of different bioabsorbable interference screws: A report of six
cases [case report].
Arthroscopy 13:
238–244, 1997
47. Stapleton TR, Waldrop JI, Ruder CR, et al: Graft fixation strength with
arthroscopic anterior cruciate ligament reconstruction. Two-incision rear
entry technique compared with one-incision technique.
Am J Sports Med
26:
442–445, 1998
48. Steiner ME, Hecker AT, Brown CH Jr, et al: Anterior cruciate ligament
graft fixation: Comparison of hamstring and patellar tendon grafts.
Am J Sports Med 22:
240–247, 1994
49. Suh JS, Jeong EK, Shin KH, et al: Minimizing artifacts caused by metallic
implants at MR imaging: Experimental and clinical studies.
AJR Am J
Roentgenol 171:
1207–1213, 1998
50. Swenson TM, Harner CD: Knee ligament and meniscal injuries: Current
concepts.
Orthop Clin North Am 26:
529–546, 1995
51. ToutoungiDE, LuTW, LeardiniA,et al: Cruciateligament forcesinthe human
knee during rehabilitation exercises.
Clin Biomech 15:
176–187, 2000
52. Weiler A, Windhagen HJ, Raschke MJ, et al: Biodegradable interference
screw fixation exhibits pull-out force and stiffness similar to titanium
screws.
Am J Sports Med 26:
119–128, 1998
Vol. 29, No. 4, 2001 Bioabsorbable versus Metal Screws for ACL Reconstruction 425
