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:

1. 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.
2. 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.
3. 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 inaccurate.
4. 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).
5. 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.
6. 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?
7. 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 formation.
8. 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?
9. 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.
10. 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 inaccurate.
11. 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.

Yours sincerely

Dr. Andreas.....