Response to Letter Regarding Instant Cage Making Using PMMA

First, we would like to thank the authors for their letter that addresses the problematic legal issue associated with the use of polymethylmethacrylate (PMMA) in spinal surgery.

Our published study was designed from a scientific point of view without commercial interest. The project's main objective was to develop an optimal design of a formed piece of PMMA, attempting to overcome known drawbacks such as subsidence and nonfusion of an unformed dowel of PMMA.

Biomechanical tests of our PMMA cage versus a commercial polyetheretherketone (PEEK) cage (Rabea, Signus Medizintechnik, Alzenau) showed comparable properties. Having obtained local ethics committee approval and on registration at ClinicalTrial.gov (trial ID NCT01607775), we conducted a prospective randomized clinical study comparing our designed PMMA cage with a PEEK cage.

A total of 38 patients enrolled in the trial received 47 PMMA cages. Through to the end of follow-up at 1 year, no incidences of implant-related complications were recorded. Clinical and radiologic results were not inferior to that of the commercial PEEK cage (publication in review). The PMMA cage was used only within this study.

As commented by the authors, legal policies for PMMA use have not been decided definitively. Although the German Medical Devices Act covers PMMA, to our knowledge its use as an intervertebral spacer or vertebral body replacement is not further regulated. An unformed dowel of PMMA that is individually designed for each patient could be interpreted as a custom-made product and does not require special legal regulations. However, on applying a more sophisticated approach of standardized design, allowing commercial device use, legal requirements for Conformité Européenne marking of medical devices apply, complicating device development and manufacturing.

Due to this situation we did not want to use our designed PMMA cage in our daily clinical routine. With regard to the good results of our clinical study, we would very much appreciate a clarification of legal terms regulating the use of PMMA cages in spinal surgery. Considering the current low price of PMMA cages, companies are less motivated to solve this problem.