SAW BLADE FOR TIBIAL PLATEAU LEVELING OSTEOTOMY

Abstract

The present invention provides a surgical instrument for performing closing wedge osteotomies by a single step procedure.

Claims

1. An oscillating saw blade adapted for bone surgery, wherein its thickness increases from the leading edge at its lower end towards its upper end.

2. The saw blade of claim 1, wherein its thickness increases stepwise.

3. The saw blade of claim 2 comprising cutting teeth at the leading edges of each step.

4. The saw blade of claim 1, wherein the blade is of curved shape.

5. The saw blade of claim 4, wherein the curved shape is adapted for use in a Tibial Plateau Leveling Osteotomy TPLO procedure.

6. The saw blade of claim 1, wherein the blade thickness at the leading edge is uniform while the blade thickness increases in steps away from the leading edge but not along its longitudinal edges.

7. The saw blade of claim 6, where the shape of the cross-section at the trailing end is of biradial, Slocum type, with the radius of curvature of the concave surface equal to the radius of curvature of the convex surface.

8. The saw blade of claim 1 wherein the blade is planar.

9. The saw blade of claim 2 wherein the steps are in the range of 0.1 to 1.0 mm, preferably in the range of 0.3 to 0.5 mm, most preferably about 0.4 mm.

10. The saw blade according to claim 1 wherein the angle corresponding to the increase in thickness is in the range of 2 to 10 degrees, preferably about 5 degrees.

11. A Tibial Plateau Leveling Osteotomy (TPLO) procedure, comprising performing a TPLO procedure with a saw blade of claim 1.

12. A planar closing wedge osteotomy procedure, comprising performing a planar closing wedge osteotomy with a saw blade of claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 shows perspective views of a curved TPLO saw blade from outside and inside.

[0027] FIG. 2 shows four orthogonal views of the TPLO saw blade.

[0028] FIG. 3 shows cross-sections of the TPLO saw blade near its leading edge and at the level with the fully developed biradial feature.

[0029] FIG. 4 is a longitudinal cross-section of the TPLO saw blade.

[0030] FIG. 5 shows a detail of the TPLO saw blade.

[0031] FIG. 6 shows the medio-lateral view of the tibia and its cross-section in the frontal plane with a completed TPLO cut.

[0032] FIG. 7 shows a perspective view of a planar, closing wedge oscillating saw blade.

[0033] FIG. 8 shows orthogonal views of the planar, closing wedge oscillating saw blade.

DETAILED DESCRIPTION

[0034] This invention is based, at least in part, on in vitro experiments and clinical observations that have helped us identify the fundamental causes of the mechanical failures of the TPLO procedure.

[0035] Our experimental work with dog cadavers, with the hind leg loaded in a testing machine in compression between the femoral head and the hock joint, has shown a strong tendency of the TPLO constructs to a valgus drift at the stifle. Intact legs do not show that tendency and on average need twice the load to failure in comparison to TPLO. With the stifle moving into valgus position the load across the stifle joint shifts laterally and hence creates even higher loading of the weakly supported lateral side. The cortices on the lateral side cut into each other. The TPLO plate is fixed on the medial side and the collapsing support on the lateral side places the plate under bending and the screws under bending and pullout forces. The critical fixation is usually proximally with the weaker, cancellous bone for screw anchorage. Proximal screws are also placed at a short distance to each other. This can result in a pullout of the screws or ripping out of a larger piece of bone from the proximal tibia. Another consequence of the lateral collapse is occasionally observed fracture of the fibula, which is of some real clinically negative impact.

[0036] A moderate closing wedge of preferably about 5 degrees on the medial side of the cut results in the medial shift of the joint load sparing the lateral cortices overload. As a practical consequence, the proximal screws pullout loading is much reduced, paving the way for using mono-cortical screws in both proximal and distal segments. This can spare the time in surgery but also reduce some risks of drilling for and using bi-cortical screws. The most cranial screw in the proximal segment of TPLO in many cases exits on the lateral side just under the tendon of the long digital extensor. A long screw can cause damage or even rupture of this tendon.

[0037] In many instances, the most caudal screw line of insertion is very close to hitting the fibula. In our in vitro experiments, dissections have shown a number of cases where the drill bits have done damage to the fibula due to uncontrolled penetration of the drill bit past the lateral cortex. This was observed with the surgeries performed by very experienced surgeons and different plate designs.

[0038] The further benefit of using mono-cortical screws is the reduced cost of implants with a greatly reduced inventory of the screws of different lengths.

[0039] FIG. 1 shows two perspective views of the curved TPLO saw blade 100 - the view (a) is from the outside, convex side 1 of the saw blade; the view (b) is from the inside, concave side 2 of the saw blade. The leading edge of the saw blade is provided with fine cutting teeth 3. Up to about a half of the saw blade length, the outside face 1 of the saw blade 100 is provided by stepwise distributed cutting teeth 4 formed by flat-ended cylindrical mill cutting the steps of the saw blade down to just below the surface of the step distal to the level of the teeth (see FIG. 5). For easier removal of the bone debris, a number of windows 5 are cut through the full thickness of the saw blade. Thickness of the blade along the longitudinal edges 11 is constant and equal to the thickness at the leading edge.

[0040] A conventional dome-shaped roof 6 of the saw blade connects the blade to the hub 7 used to attach the blade to the oscillating saw machine.

[0041] FIG. 2 shows four orthogonal views of the saw blade according to the present invention.

[0042] FIG. 3a shows the blade shape at the leading edge. The radius R defines the shape of the proximal tibia segment after the osteotomy is performed. The radius on the outside of the saw blade leading edge is (R + t) where t is the thickness of the blade. The lateral side of the TPLO osteotomy of the distal segment will thus have this slightly larger radius and thus will not perfectly match the proximal segment. However, as already mentioned, on the lateral side the cortices will cross at only two points and this mismatch is of minimal consequence.

[0043] FIG. 3b shows the blade shape at the end of the stepped, cutting part of the saw blade. With careful design, it is possible to make the radius of the convex side be identical to that of the concave side. This is at approximately the level of the medial cortex when the osteotomy is completed. TPLO rotation here results in much longer potential contact and having the radii of the bone segments match provides some advantage to bone healing as advocated by Slocum.

[0044] FIG. 4 shows a longitudinal cross-section of the blade. Steps 8 reduce the thickness T of the blade in the mid-plane at the proximal end of the cutting features, to the thickness t at the leading edge. Cutting teeth 4 are formed along the leading edges of steps 8. This results in an approximate wedge-shaped cut of an angle 9. The blades can be produced with angles in increments of 2 to 3 degrees, within the range of 2 to 6 degrees. The saw blade can be manufactured with steps 8 of 0.1 mm to 1.0 mm, depending on the size of the blade, particularly its radius of curvature R, and the desired wedge angle 9. One of the most commonly used TPLO blades has the radius of curvature of 24 mm. For that blade and the wedge angle of 5 degrees, a practical step size is 0.4 mm with the thickness at the leading edge of 0.7 mm.

[0045] FIG. 5 shows a detail of the saw blade at the leading edge with the teeth 3, the openings 5 and the outside cutting teeth 4 along the leading edges of the steps 8.

[0046] FIG. 6a shows the medio-lateral view of the proximal tibia 300 with the TPLO osteotomy 301. The proximal segment 302 is rotated by an angle 303 to reduce the instability due to the failure of the cranial cruciate ligament. FIG. 6b shows a cross-section of the tibia in the frontal plane after the osteotomy is performed by the blade of the present invention. The resulting wedge 9 of bone removed will be closed by application of the TPLO plate, hence the term closing-wedge osteotomy. The compressive joint reaction 310 will be shifted medially to 311, reducing the loading of the critically weak support 312 on the lateral cortex.

[0047] FIG. 7 is a perspective view of the planar saw blade 200 with the same features of the TPLO saw blade disclosed above. The blade cuts by oscillating movement 202 around the axis 201. The leading edge of radius R is provided by cutting teeth 203.

[0048] FIG. 8 shows orthogonal views of the planar saw blade 200. The blade thickness is increasing by steps 208 from the leading edge thickness t to the end thickness T. This results in an approximate wedge angle 209. For practical applications in bone surgeries, the blades can be manufactured with the wedge angle in 2 to 3-degree increments, with the range covering 2 to 10 degrees. Windows 205 facilitate bone debris removal produced by the leading edge teeth 203 and the trailing, step-to-step, teeth 204.

[0049] Having disclosed at least one embodiment of the present invention for a TPLO saw blade and a planar oscillating blade, variations will be understood by one of ordinary skill in the art. Such adaptations, modifications, and improvements are considered part of the invention.