Effects of sagittal plane prosthetic alignment on standing trans-tibial amputee knee loads

The influence of sagittal plane prosthetic alignment changes on loads appli ed to the ispilateral knee was investigated using 5 trans-tibial amputee su bjects. The goal was to determine which prosthetic alignment results :in th e most energy efficient standing and also minimises stresses on knee struct ures during standing. The electromyogram, the external mechanical loading of the prosthetic leg a nd the amputees' posture were recorded for a wide range of prosthetic align ments. The EMG of the vastus lateralis and biceps femoris muscles was measu red bilaterally; the EMG of the gastrocnemius muscle was measured only on t he contralateral side. The distance between the anatomical knee centre and each individual's load line, as determined by the Otto Pock "L.A.S.A.R. Pos ture" alignment system, was used as a measure of the mechanical load applie d to the knee joint. Prosthetic alignment has almost no influence on muscle activity of the cont ralateral lower limb during static standing. On the other hand, prosthetic alignment has a significant influence on the load applied to the amputee's ipsilateral knee joint. The external knee moments applied to the knee ligam ents and knee muscles on the amputated side change systematically in respon se to different plantar flexion or dorsiflexion angles of the prosthetic an kle-foot. During standing the extensor muscles stabilise the limb by contra cting if the load line is located less than 15mm anterior to the anatomical knee centre. The biceps femoris muscle appears to have little or no protec tive function against hyperextension during standing even if large external knee extension moments are caused by excessive plantar flexion. Such extre me alignments significantly increase the stresses on knee ligaments and the posterior knee capsule. When prosthetic sagittal plane alignment is altered, the trans-tibial amput ee compensates by balancing the upper part of the body over the centre of p ressure of the prosthetic foot. Biomechanically optimal alignment of the trans-tibial prosthesis occurs whe n the individual load line is approximately 15mm anterior to the anatomical knee centre, permitting a comfortable, energy efficient standing and minim ising the mechanical loading on the knee structures.