Research in Spinal Surgery using Meccano

by Michael Adler FRCS, Orthopaedic Surgeon

A research project was set up some years ago in the Spinal Unit of the Meir Hospital in Israel, affiliated to Tel Aviv University, to examine whether a new type of surgical correction for certain types of spinal deformity could be achieved. 

The condition for which the research was carried out is called Scoliosis.  This is a curvature of the spine, and can be caused by congenital abnormalities, infection or degeneration  - but the largest group is called idiopathic and has no known cause.  For unknown reasons it nearly always affects young growing girls, and it can progress to an alarming degree and produce a severe deformity if not spotted in time and arrested or corrected.  The normal spine has its own natural and normal curvature, namely a thoracic kyphosis with a convex curve at the back, and lumbar spine lordosis with a concave curve at the back.  Viewed from the front, there should no curvature.

In scoliosis, the spine seems to buckle into a complex curve, just as a piece of rubber tubing would if its two ends were approximated.   In a scoliotic spine, when an x ray of the spine is viewed from the front, it usually has an S-shaped curve, and there is a strong rotation of individual vertebrae.  Viewed at right angles from the side, the physiological curves might be greatly exaggerated or decreased and the diagram shows this.  Fig. 1 shows the deformity on antero-posterior and lateral views.

 Fig 1.  Xrays back and side views to show scoliosis deformity

At an early stage in the deformity, preventing the curve from increasing might be achieved with a body brace.  Many cases however proceed despite adequate treatment to a stage where surgical correction is required.  This treatment takes the form of internal fixation of the spine with corrective rods and hooks, which are attached to key vertebrae, and then the spine correction achieved by applying distractive and apical forces.  When as much correction is applied as is thought to be safe, the spine is fused with bone graft, so that the vertebrae themselves become linked to each other by new bone growth, and will no longer require the rods and hooks.  These could even be removed at a later date. 

This explanation provides the background to further research into the possibility of applying a corrective frame, or external fixator, temporarily to the key vertebrae during surgery before this internal fixation was applied.  It was hoped in this way that a much better and safer correction of the deformity could be achieved.

Prototype External Instrumentation apparatus

An apparatus was designed according to the mechanical requirements.  This can be seen in Fig. 2 and shows a distraction system, a method to apply a force at the apex of the curve, and a means to correct any rotational element.    It can be appreciated at once that this implies a three dimensional correction.  It was thought that using such an apparatus intra-operatively might achieve a far better correction than conventional methods.  The device was given the name of  ISIS or Intra-Spinal Instrumentation System.

Fig 2.  External Fixation apparatus

In the drawing, three groups of square blocks can be seen.  These represent the upper and lower strategic vertebrae at the upper and lower part of the apparatus, and the intermediate apical vertebra group.  Attached to the blocks are hooks, with which a purchase or fixation is obtained on the vertebrae.  The direction of the various forces applied in three dimensions is shown by the arrows. 

The Distractor or Horizontal Spreader (A) is applied to the upper and lower strategic vertebrae.  The application point is a universal joint - ball and socket (B) - which is one face of a metal block (C) attached to the Transverse Linking Perforated Plate (D) of the instrumented vertebra.

The Transverse Distraction device (E) is applied through a hinge joint (F) on the metal attached to the apical vertebra.  It is held in a sliding collar attached to the longitudinal bar of the Distractor.

The rotation and Kyphosis Device is a combined lever (J) attached rigidly to the metal block fixed to the transverse linking plate of the apical vertebra.  The position of the lever is maintained by fixation to the quadrant (G) held in the longitudinal rod (H) free to rotate in bearings, which can be attached to the apical vertebra.

The Meccano apparatus closely follows the design of the prototype (Fig 3).  The spinal model was a life size plastic replica.   The fixation to the three vertebrae can be seen.  The fixation is to the transverse processes of each vertebra, one on each side.  1/2" x 1" angle brackets grasp the transverse processes and are bolted to threaded couplings.  The threaded couplings are joined to each other by transverse double arm cranks.   

Fig 3.  The Meccano scoliosis external fixation apparatus

The 11 1/2" distraction rod stretches the ends of the curve apart.  It is applied to the end vertebrae through 2" rods and couplings.  The apical vertebra can be moved sideways by a 4" horizontal rod which runs from the distraction rod to the central apical vertebra.

The apical vertebra can also be moved upwards or downwards by a vertical 3 1/2" screwed rod.  The screwed rod is attached to the apical vertebra  and runs upwards through a quadrant made from 4" curved strips.  The quadrant is itself held between the end vertebrae by a rod.  Two wheel flanges each with a threaded crank, one above the curved strips and one below, rotate on the screwed rod, and can alter the position of the apical vertebra upwards and downwards, but the rod can also correct rotation by being moved through the quadrant.

Conclusion

There was no need to build a prototype, thus saving a great deal of money.  It was instead built in Meccano.  Rather than demonstrate the system with drawings, the Meccano apparatus was used to prove the concept of a dynamic external corrective spinal fixation system, and to demonstrate the concept in a practical way. The principle of temporary external fixation during spinal surgery is new, and will be developed further.  It is noteworthy that Meccano was easy to use in this unique application.  This was especially so when Meccano required fixation to individual vertebrae.  Once this had been achieved, it was a necessary to build a system which could replicate the prototype.  It is deeply satisfying to think that Meccano could be used in this type of research because of the ready supply of parts which could easily duplicate the engineering requirements of the system without any machining, modification or addition of complementary components.  

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