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Biomechanical Analysis Of A New Generation Posterior Derotation system : The One Lock Spine Screw Rod System

Swapnil M Keny*, Ram Chaddha*, S Vasudevan**,  Shrikrishna Joshi***

* Department of Pediatric Orthopaedics and Pediatric Spinal surgery, Gurunanak Hospital, Mumbai, India
**Department Of Mechanical Engineering, Indian Institute of Technology, Powai, Mumbai, India
*** Department of Biomechanical Engineering Adler Mediequip, India

 Address for Correspondence

Swapnil.M. Keny,
Department of Pediatric Orthopaedics and Pediatric Spinal surgery,
Gurunanak Hospital,

J.Orthopaedics 2007;4(1)e2


A plethora of Spinal Screw Rod systems are available as of today. However most if not all have issues as regards closure mechanism and biomechanics. Though these issues may seem to be of trivial importance during surgery, the long-term effects of such implantations may be catastrophic[13]. Other than addressing these issues, the One Lock Screw Rod System, by virtue of its indigenity, also addresses the issue of high cost. The design objectives of the One Lock screw rod system are high closure security, a low fiddle factor and a high resistance to bending loads.

Material and Methods :

The entire study was conducted in the Department of Mechanical Engineering at the Indian Institute of Technology, Powai  Mumbai .The central research committee of the Indian Institute of technology approved the study.

The study and testing of the system was carried out as per the standard ASTM F:1798 – 1997, “Standard Guide for evaluating the static and fatigue properties of interconnection mechanisms and sub-assemblies used in spinal arthrodesis implants”.

The study was performed using a Universal testing machine (UTM) . The automated 50 kN UTM, [Lloyds U.K - Model no EZ 50] was connected to a Windows based software, [National Instruments, U.K] providing continuous load displacement graphs. Four variants of the One Lock were tested initially. Since the MUV variant was found to be the most superior amongst all the variants, the final tests were performed on this variant only.

For the purpose of accuracy, the Zero error which is minimal in the automated UTM was calculated for the particular machine and was entered into the computer programme .This would nullify all the false negative values. A metallic fixture was designed to hold the MUV variant. This in-turn perfectly reciprocated with the mechanical levers of the UTM. The design of the fixture assured a snug fit, minimizing the toggle effect between the implant and the fixture. In order to rule out technical errors, each of the tests was repeated six times and only the average readings were reported.

Axial Loading Test ( Picture 1,2,3)

Assembly of the screw rod unit was performed before mounting . A Universal Torque screw driver was used for tightening the nut ensuring uniform tightening of the inner set screw to the recommended value. After vertical mounting, a graduated force was applied to the assembly in the direction parallel to the rod with the force nucleus being the centre of curvature of the rod. The force tried to simulate and exceeded the actual environment of the axial forces which are transmitted to the screw rod interface. The Force at which there was a uncoupling of the screw – rod assembly was assessed and was digitally plotted ( Figure 1).

Torsional loading Test ( Picture 4,5)

Similar to Axial loading test, a screw rod assembly was coupled. The assembly was mounted horizontally within the fixture and a graduated force was applied at a point as defined in the test method. The loading points were accurately measured and pre-marked with a metallic marker. This was done in order to simulate the point at which the actual forces precipitate when such a construct is used as a posterior spinal instrumentation in the human spine. The Force at which there was a uncoupling of the screw – rod assembly in torsion, was assessed and was digitally plotted similar to the previous described test (Figure 2)

Bending Moment Test ( Picture 6,7)

An screw-rod construct assembly, similar to the construct used as a posterior spinal instrumentation in the human spine was created. This construct was mounted vertically on a specially designed fixture which simulated the conditions of Flexion and extension as would occur on the human spine in vivo. A controlled force was applied in a graduated manner at a pre-marked point on the screw as defined by the test method. The force at which there was a mechanical failure of the construct leading to deformation of the construct was assessed and the graph was plotted.

Head Spread ( Picture 8,9)

To study Splaying of the head after torque tightening of the screw, the screw – rod assembly was coupled and the Nut was tightened with a torque screw driver upto the recommended tightening torque as is routinely done in the last step of final tightening. The splaying of the head was measured with the help of Digital calipers as shown in the picture.

Results :

Axial Load Testing
The OneLock spine screw-rod construct was found to have an axial slip load of 3222 Newtons.
 Fig 1 Depicts the Load at which axial slip occurs at the screw rod interface
Torsional Loading Test
The Torsional Slip Load of the OneLock screw-rod construct was found to be 9.61 Nm.

Figure 2 Depicts the Load at which Torsional slip occurs at the screw rod interface

Flexion Extension Moment Test

The flexion extension moment of the OneLock screw-rod construct was found to be 23.5 Nm

Figure 3 depicts the flexion extension moment


Head Spread
The Head Spread of the MUV variant was 0.05 mm at recommended Tightening Torque.

Discussion :

Pedicle screws have been used for instrumentation of the spine since the mid eighties. With the passage of time and refinements in the field of  Biomedical engineering, understanding of the biomechanics of pedicular screw rod systems have significantly improved [1,3,4,6,7,8].
A review of the currently available systems revealed certain issues regarding implant biomechanics and closure mechanism at the screw rod interface.
The conventional Double Locking Screw Rod interface systems have a three step closure. Approximation of the rod to the screw by applying the inner nut, reinforcement of the locking by an outer nut and in the final step, re-tightening of  the inner nut with a Torque Screw Driver. The primary issues faced by surgeons with these systems were  Cross threading of the inner nut and a hindrance to loading of the outer nut when the inner nut was fully tightened. These issues made the the use of Double locking systems time consuming and cumbersome especially during surgeries for spinal deformity corrections.
The earlier available versions of the single locking screw-rod interface systems had bulky screw heads which lead to prominence when used in children and lean individuals. These earlier locking mechanism designs of the inner nut did not provide adequate gripping force thus leading to occasional interface failure[2,4,]. Additionally head splaying was evident during the final ‘torque’ tightening which further exacerbated the problems. Though the subsequent improved versions promised to address these issues, the issue of head splaying, observable in various well known systems, still remained pertinent. The reason for persistence was that, on tightening the locking screw, the dissipation of the  horizontal vector of  force applied leads to splaying of the head.
Some versions of reduction screws used in Spondylolysthesis have a bulky head. They also have an indirect locking mechanism and need approximation of the tabs which is an additional procedural step.
The variable designs of the tapered screws have issues of removal, since once the screw head is locked, it cannot be unlocked. Additionally head splaying is evident when he inner nuts are tightened. Screws with multiple components require to be assembled on the table and are not preferred for load-bearing applications such as a lumbar spine construct[14,15,16]. The lateral offset type of polyaxial screws have a fiddly locking mechanism which be cumbersome during usage.
In summary most systems have ‘ Closure Mechanism’issues. Systems with low ‘fiddle factor’ have a low connection security and those with a high ‘fiddle factor’ offer a high connection security.
The OneLock system was designed to address these issues. The Torsional slip of the One Lock System was 9.61 Nm which is better than most of the currently available systems. The flexion Extension moment, a critical indicator of performance in a real life load bearing construct, at 23.5Nm is also superior to most of the available systems.
As regards head splaying, the OneLock strives to achieve the Paradigm shift,  that is to prevent splaying in the first instance.The mechanism by which this is achieved is the square threaded design of the inner set screw combined with the proprietary design of the screw head profile. With the square threads, tightening the inner screw transmits 100 % of the torque axially to the rod without dissipation. The proprietary design of the screw head profile absorbs residual outward forces. Hence unlike an standard thread of many known systems, there is no resultant dissipation of the tightening torque. This leads to minimization of head splaying. Compared to the best contemporary designs (0.11 mm), there is a 50% reduction (0.5 mm ) in the head splay at recommended tightening torque.

The OneLock pedicle screw rod system is a easy to use Single step closure system with a best in the class head spread, a torsional grip strength higher than most known systems, a bending moment resistance higher than most known systems, an axial grip strength higher than most known systems at comparable and competitive cost levels.


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This is a peer reviewed paper 

Please cite as : Swapnil M Keny:Biomechanical Analysis Of A New Generation Posterior Derotation system : The One Lock Spine Screw Rod System

J.Orthopaedics 2007;4(1)e2





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