Retrospectively to analyse factors contributing to the development of hyperextension deformity after distal surgical lengthening of hamstrings in cerebral palsy. In the cohort of 51 diparetic patients (98 operated knees ) surgically treated for fixed flexion deformity at least five yers before this study was contemplated, the range of of hyperextension of the knee was measured.

According to surgical technique two subgroups were differentiated:

A./ Simple cutting of gracilis and semitendinosus, followed by fractional lengthening of semimembranosus and biceps femoris.

B./ Proximal stumps of gracilis and semitendinosus after its transverse division were anchored to fractionally lengthened semimembranosus.

Assessment involved: Measurement of hyperextension of the knee in lying and standing position and by walking using video-documentation. Values of Bleck popliteal angle before and after operation were estimated. This cohort did not involve any case with residual fixed plantar flexion of the foot. Both surgical subgroups were compared for occurence of hyperextension deformity > 5° in lying, standing positions and by walking. Testing by Fisher exact test did not show any statistical difference in all three compared situations (p > 0,05). The occurence of hyperextension > 5°in lying position was found in 5 knees ( 5,1 %), in standing position in 8 knees ( 8,2 %) and by walking during stance phase in 12 knees (12,3 %). In no case hyperextension of the knee exceeded 15°. Statistical testing between the postoperative Bleck popliteal angle ( stratified into classes below 20° and over 20°) and the occurence of hyperextension deformity did not show in Fisher exact test any statistical significance. On the other side the testing between postoperative Bleck popliteal angle ( stratified as above) and the type of surgery showed statistical significancy in Fisher exact test. Simple cutting of gracilis and semitendinosus brought about oftener the lower values of Bleck popliteal angle below 20°.

It seems that the role of type of treating superficial flexors in the ways used in this study was not so much decisive for development of hyperextension deformity.