Dear Dr. P..
I read with interest the current case report "Tibial and pretibial
cyst formation after anterior cruciate ligament reconstruction with bioabsorbable
interference screw fixation" reported by Vladimir Martinek and Niklaus Friederich,
published in the March 1999 issue of The Journal (REF)
I have some questions and comments:
- On page 1 (para 1, line 6) the authors cited Kurosaka et al. with respect to
complications associated with metal interference screws. Kurosaka performed a
biomechanical study. This reference is inaccurate.
- On page 1 (para 1, lines 13 and 17) the authors cited Bach with respect to the use of
biodegradable interference screws. Bach et al. reported complications associated with
metallic interference screws This reference is in accurate.
- On page 2 (para 1) the authors mentioned that a 6 x 23 mm interference screw was used.
This specific screw is available only in one size (8 x 23 mm, This statement is
- The authors recognized a fluctuant mass over the tibial tunnel aperture site. Why did
they not perform a diagnostic puncture to be sure that the diagnosis of a foreign-body
reaction was correct and the subsequent treatment appropriate? The reported foreign-body
reaction, if the diagnosis were correct, would be classified as a grad II (EA-2) reaction
and should not require extensive debridement (Hoffmann).
- The authors described a large osteolytic lesion as shown in the radiograph (Figure 1).
As seen in the figure, this lesion is located proximal to the bone plug directly in the
subchondral bone. Additionally, the MRI (Figure 2) showed the maximum cyst formation
distal and proximal to the bone plug with no evidence of plug resorption or tunnel
osteolysis at the former screw site. It has been described in several reports that
osteolysis accompanying biodegradable implant degradation is maximally pronounced directly
at the former implant site (Weiler, Böstmann, Hoffmann). Furthermore, in Figure 3 (the
legends to Figures 3 and 4 are misplaced) the imprints of the screw threading within the
bone are clearly visible. Thus there is strong evidence, that the osteolytic lesion is not
directly related to the degradation of the screw, because the osteolysis is not maximally
pronounced at the former implant site; if an osteolytic lesion in this case were due to
the screw material, the imprints of the threading would be resorbed at this site.
- The authors wrote that the biodegradable screw was not detectable on the MRI as seen in
Figure 2. They further wrote, that during revision a remaining part of the screw head was
found inside the tunnel. It is well known that solid fragments of poly-(L-lactide)
implants are clearly visible during degradation as hypointense structures in MRI (T1)
(Pihlajamäki 2x). In our lab, we also found that the degradation process of
poly-(D,L-lactide) in a MRI follow-up study is strongly correlated to histological
findings; and if solid fragments are left, they are clearly visible on the MRI. Thus, it
seams that there are inconsistencies between their clinical and radiological findings. Do
the authors have any explanation for this?
- No connection between the tunnel cavity and the knee joint was found by the authors. How
did they prove this? This is a crucial point for an adequate discussion of the case
because, as the authors already mentioned, pretibial cyst formation can also occur with
metal implants (Brettler, Fleiss, Johnson, Liu, Roesner, Simonian, Victoroff). There are
various explanations for such cyst formation, such as incomplete graft incorporation,
tissue necrosis, or ganglion-like cyst formation. To clearly state that a foreign-body
reaction directly related to the screw material has been found, the authors should have
excluded that the origin of the cyst was related to the above mechanisms. Because this was
not done, the discussion should have included this alternative mechanism of cyst
- During histological examination no refractile material or inflammatory response was
found. It is generally accepted, and has been proven in several experimental and clinical
reports, that sterile sinus formation due to biodegradable implant degradation is
associated with a severe inflammatory response, such as macrophage activation and
foreign-body giant cell formation (Weiler, Hoffmann, Hoffmann, Böstmann, Böstmann,
Santavirta). Therefore a histologically proven foreign-body reaction would benecessary to
conclude that such a reaction was due to sterile sinus formation accompanying end stage
implant degradation (Hoffmann).Do the authors have any explanation why no inflammatory
response was seenhistologically? Sterile sinus formation and the development of osteolysis
accompanies final implant degradation, due to a limited clearing capacity of the
surrounding tissue at this time (Hoffmann, Weiler, Böstman, Casteleyn). Do the authors
have any evidence that eight months is the time of maximum implant degradation for this
specific poly-(D,L-lactide) interference screw? Furthermore, no implant remnants were to
be found in the histology. Because this foreign-body reaction is directly related to
implant degradation it is essential to demonstrate the presence of implant remnants within
the inflammed tissue to make the conclusion the authors did (Weiler, Hoffmann). Even if an
amorphous implant was used, polymer particles should be found histologically within
macrophages or foreign-body giant cells. The only possible reason why no implant remnants
were found, is that an inappropriate histological preparation technique was used. But even
in this case, the authors should at least have found foreign-body giant cells and
macrophages. How did the authors prepare the tissue for histological evaluation?
- On page 319 (para 1, line 4) the authors cited Bach concerning clinical experience with
poly-(L-lactide) interference screws. Bach reported pitfalls associated with metal
interference screws. This reference is inaccurate.
- On page 319 (para 1) the authors referred to foreign-body reactions to polydioxanone and
"lactide-glycolide" polymers. In none of the cited references were these
materials used. Friden et al. and Böstman et al. used polyglycolide implants, and Edwards
et al. focused on polyglycolide-co-trimethylenecarbonate implants. This reference is also
- The authors further described that no osseous substitution had taken place for up to
eight months in the reported case. Osseous replacement of a biodegradable implant may
require up to several years after complete implant degradation (Weiler), and should thus
not be expected so soon in this specific case.
In summary, the authors can not reasonably contend that cyst formation
may not also have occurred if a metal implant had been used. Additionally, they were not
able to demonstrate any inflammatory response histologically. Furthermore, the authors
clearly showed in their figures that the maximum osteolytic lesion was not directly found
at the former implant site. Thus their findings are not strong enough to classify the
reaction as a foreign-body reaction directly related to the screw material. Although such
a reaction could theoretically occur with any biodegradable implant, reasonable proof is
lacking to reach the conclusion asserted by these authors.