Does More Hardware Mean More Stability at L5–S1?

Fusion is a biological process, but it begins with a mechanical challenge.

Before a solid fusion develops, the treated segment must be kept sufficiently stable to limit unwanted motion. Posterior fixation provides that support, and at a mechanically demanding level such as L5–S1, achieving reliable immediate stability becomes an important part of the construct.

Traditionally, greater fixation has often meant more instrumentation—additional screws, bilateral fixation, and connecting rods designed to increase rigidity. But every fixation strategy comes with a broader surgical consideration: how much instrumentation is actually necessary to achieve the stability the segment requires?

That question becomes particularly relevant as spine surgery moves toward approaches that aim to achieve the surgical objective while limiting unnecessary exposure and instrumentation.

Could a different fixation strategy provide comparable immediate stability without relying on the same amount of posterior hardware?

A cadaveric biomechanical study published in the Journal of Spine Surgery offers an interesting look at that question by comparing different posterior fixation strategies at L5–S1. 

What Happens When the Constructs Are Put Under Load?

The investigators studied 14 fresh human cadaveric lumbar specimens and compared three configurations: bilateral transfacet pedicle screw (TFPS) fixation using two screws, bilateral pedicle screw-rod (BPSR) fixation using four screws and two rods, and unilateral pedicle screw-rod (UPSR) fixation using two screws and one rod.

All screws were 5.0 × 40 mm, providing an equivalent-size comparison. The constructs were tested at L5–S1 during flexion-extension, lateral bending, and axial rotation to assess how well each restricted motion. 

The difference between bilateral TFPS and unilateral PSR was consistent. TFPS produced significantly less ROM in all four tested directions: flexion (P=0.001), extension (P<0.001), lateral bending (P<0.001), and axial rotation (P<0.001). 

But the comparison with bilateral pedicle screw-rod fixation was more revealing.

TFPS demonstrated lower mean mobility across all loading directions. Compared with bilateral PSR, ROM was significantly lower in extension (P=0.048) and axial rotation (P=0.04). Differences in flexion (P=0.06) and lateral bending (P=0.052) were not statistically significant. 

In other words, the construct using fewer components did not show a corresponding loss of immediate stability.

Source of image: https://pubmed.ncbi.nlm.nih.gov/39399093/ 

Why the Fixation Path May Matter

The findings shift attention from simply how much hardware is used to how that hardware stabilizes the segment.

Transfacet fixation crosses the facet joint and engages the pedicle, whereas pedicle screw-rod fixation stabilizes the segment through pedicle screws connected by rods. The authors suggest that the downward TFPS trajectory and lag-screw design may create compression across the facet, potentially contributing to the observed stability. 

The loading results also show why construct mechanics cannot be reduced to screw number alone. During lateral bending, for example, the authors note that TFPS sits closer to the coronal axis of rotation, whereas the more lateral position of pedicle screw heads provides a larger lever arm for resisting motion. 

Different constructs, therefore, may reach the same goal through different biomechanical pathways.

What Should We Take From the Comparison?

The study does not establish that one fixation strategy should replace another.

It measured immediate stability in cadaveric specimens, not fusion rates or clinical outcomes. The sample was small, the TFPS specimens were younger despite comparable bone mineral density, and both constructs used 5.0-mm screws—an experimental choice that may not fully reflect pedicle screw dimensions used clinically. 

Technical execution also remains relevant. Among 14 TFPS screws, there were no inferior facet fractures, but one medial canal breach occurred and one specimen had facet engagement on only one side. 

What the study does provide is a useful biomechanical perspective on the question raised at the beginning.

At L5–S1, the amount of instrumentation alone did not determine immediate stability. A bilateral transfacet construct using two screws controlled motion comparably to the four-screw bilateral pedicle screw-rod construct under the conditions tested.

For posterior fixation, that shifts the conversation from “How much hardware is needed?” toward a more useful question:

How effectively does the chosen construct control the motion that matters?

That, rather than implant count alone, may be the more meaningful way to think about immediate stability.

Interested in Reading the Full Study: https://pubmed.ncbi.nlm.nih.gov/39399093/