Four Sharp Edges to Stop a Spinal Rod Turning How Grip Grooves Replace Friction With Bite

This application covers a spinal implant whose rod seat has been cut with two parallel grooves, so that a clamped rod is held by four sharp edges rather than by flat friction. Claim 1 is an implant with a rod-receiving recess, a pair of grip grooves running along it, and a movable retaining member that pushes the rod onto the four groove edges. It was filed by Medos International Sarl, a Swiss company, and names Kevin Lee, Samuel Jacobs and Aubrey Ortiz as inventors. The application is a divisional of Australian application 2020364421.

The Problem

In spinal fusion surgery, screws are anchored in vertebrae and rods are clamped to the screws to hold the spine still while bone grows between the vertebrae. The specification stresses that the stability of this construct is what lets fusion happen. If there is too much motion between instrumented vertebrae, new bone cannot bridge the gap, and the result can be a pseudarthrosis, a “non-union” where the bone never takes over the load.

That puts a demand on every part that grips a rod: screw heads, rod-to-rod connectors and screw-head-to-rod connectors. They must resist the rod sliding, rotating or pulling out. The specification singles out transition regions, such as the join between constructs in the cervical and thoracic spine, where torsional slip on the rod is described as a serious risk. It can be caused by routine movements such as a patient twisting their head.

Space makes it harder. Room in the cervical spine is limited, the implants are small, and the specification notes they are awkward to handle in the surgical wound. Connectors have to be compact and still hold.

What This Invention Does

Claim 1 is an implant with a body that has a rod-receiving recess, open on two sides, defining a central rod axis. Part of the recess is formed by an inner surface cut with two grip grooves that run parallel to each other and to that axis. Each groove meets the inner surface at two edges, giving four edges in total, and the claim requires those four edges to lie together on a circle about the rod axis. A retaining member moves relative to the body, pushes on the rod at right angles to the axis, and presses the rod against the four edges. That engagement is what restrains the rod from rotating about its own axis.

The specification explains the difference from the prior art. In a recess with no grooves, the rod touches the inner surface along two lines, and rotation is resisted only by the clamping force and the friction it generates. With grip grooves, the rod is pressed against four sharp edges, and the specification says rotating it would require “micro shearing” of those edges at the points of contact. The grooves therefore constrain rotation in a way that requires material failure before the rod can move, so for the same clamping force the arrangement can withstand higher torque on the rod before it slips.

The specification is candid that this is a mechanical argument rather than a measured one. It contains no torque tests, test standards or comparative figures. The numbers it does give are geometric: the rod radius and the distance of each edge from the recess axis are approximately the same, so the rod sits evenly on all four edges, and a connector wing portion may be under about 5 mm, 4 mm or 3 mm in height.

Dependent claims add a gap between the two grooves so the rod can deflect slightly into it (claim 2), a separate compression member carrying the grooves (claim 4), sharp edges (claim 9), and a cross groove that splits the edges into segments and creates four corners to resist the rod sliding along its axis (claims 10 and 11). Claims 14 to 18 apply the idea to a rod-to-rod connector with two recesses and a sliding rod pusher, and claim 19 applies it to a bone screw receiver with a set screw. Independent claim 20 puts the grooves on a separate grip insert fitted into the open end of the recess.

Key Features

• Four edges on a circle. Two parallel grooves produce four edges that together define a circular radius about the rod axis, so a cylindrical rod is held at four lines of contact instead of two.
• Bite instead of friction. Turning the rod while it is pressed on the edges requires small-scale deformation of the edge, the rod surface, or both, which is the stated reason the grooves resist torque better than a smooth seat.
• A pocket and gap between the grooves. A relief between the grooves lets the rod be driven into the edges without bottoming out on the rest of the recess, so the clamping force goes to the edges and not to general surface contact.
• Segmented grooves. A transverse groove cuts across a grip groove and breaks its edges into short segments. The resulting small corners are described as resisting sliding along the rod axis as well as rotation.
• Works in connectors and screw heads. The same feature appears in a rod-to-rod connector with a sliding pusher, in a bone anchor receiver with a set screw, and in a separate grip insert.
• Material choice left open. The specification says implant components can be stainless steel, titanium or titanium alloys, cobalt chrome, PEEK, ceramics or carbon fibre, and that the edge and rod materials can be chosen so that one deforms against the other.

Who Is Behind It

The applicant is Medos International Sarl, whose US patent records give its address as Le Locle, Switzerland. It files spinal and orthopaedic patents within the DePuy Synthes group, which Johnson and Johnson MedTech describes as offering one of the most comprehensive orthopaedic portfolios in the world, spanning joint reconstruction, spine, sports medicine and trauma. Background on the group is also on the DePuy Synthes page. The three inventors are listed with Massachusetts addresses on the US counterpart.

This application is a divisional of Australian application 2020364421, and paragraph 149 of the specification says so, incorporating that earlier disclosure by reference. The specification states no priority or convention claim in its own text. From the published family record rather than from this document, the Australian parent was filed on 12 October 2020 with a priority date of 11 October 2019, which matches US application 16/599,902 filed that day. A US continuation of that family has been granted as US 12,329,420. The priority country is therefore the United States, on the strength of the family record and not of this specification.

Why It Matters

A spinal rod that creeps in its clamp is one of the quieter ways a fusion can fail. Surgeons tighten set screws to a specified torque, but the grip still depends on friction between metal surfaces, and friction is hard to guarantee after months of loading. Replacing a friction contact with a mechanical interlock is a familiar engineering move, and here it is done by changing the geometry of the seat rather than adding parts.

For a company selling spinal fusion hardware, the appeal is that the change is small. The grooves sit inside a recess that already exists, in a connector that is already compact, and the claims cover both screw receivers and rod connectors. The absence of any test data in the specification means the real-world torque benefit is asserted, not shown, but the reasoning is simple enough to follow: sharp edges that must deform before the rod can turn.

Related Concepts

• Spinal fusion – the surgery in which rods and screws hold vertebrae still while bone grows between them.
• Pseudarthrosis – the failure of bone to unite that the specification names as the consequence of instability in the construct.
• Torque – the twisting force on the rod that the grip grooves are designed to resist.
• Cervical vertebrae – the neck region where space is tight and torsional slip at transitions is a particular concern.
• Orthopedic surgery – the field in which fixation systems hold bones in a desired relationship until healing.

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AU 2026202264 was published in the Australian Official Journal of Patents on 9 April 2026 and is open for public inspection. Patent applications represent inventions that are sought to be protected and do not necessarily reflect commercially available products.