This post is a case report regarding a 2.5 year old Duck Tolling Retriever that had persistent lameness beginning within six weeks of a Helica THR, performed elsewhere. Lameness had been progressive over the six months from onset to presentation at my clinic. Response to analgesia had been poor. She had been noted to be increasingly depressed and unable to exercise for periods in excess of fifteen minutes.
Radiographs were taken and a Synovial aspirate was taken, the latter to rule out sepsis and the former to establish whether there was any evidence of peri prosthetic lucency.

There was clear evidence of lucency around the cup and in one view there was a clear gap between the osteotomised femoral neck and the base plate of the stem.

These indicated stem and cup loosening. In addition, the lateral femoral cortex appeared weakened in the region where the Helica stem had presumably been pistoning during load and unload of the limb.
Synovial aspirates revealed a hemorrhagic, non inflammatory picture and culture of fluid submitted in enhancement medium resulted in no growth of bacteria. Osteoclasts were identified in the submitted sample, presumably due to implant loosening and exposure of cancellous bone.

Problem Summary

Persistent, Progressive lameness non responsive to medical management

Evidence of Aseptic Loosening

Weakening of the Lateral Femoral Cortex in the region where a bicortical screw would be placed if a Kyon Cementless THR was performed by standard technique.

Weakening of the Lateral Femoral Cortex increasing the risk of post operative fracture.

The loose cup was of the same diameter as the largest Kyon cup that could be safely used.

Plan
To revise the Helica Hip with the Kyon Cementless THR. A smaller stem would be used than normal to allow the stem to be seated deeply enough to avoid the weakened region of the Lateral Femoral Cortex. A more distal neck cut was planned for the same reason. A 26.5 mm diameter Revision cup would be used. An ALPS (Advanced Locking Plate) would be applied following implantation to protect against post operative trochanter fracture.

Planning Radiograph

The measured lines adjacent to the Lateral cortex were to establish that there was sufficient space for three screws in the Greater Trochanter and to indicate the length of plate that would be unoccupied by screws due to the position of the prosthesis.

Surgical Findings
The Greater Trochanter fractured on simple external rotation of the femur and luxation of the hip.
At surgery the stem was noted to be loose and was easily removed:

The gap between the Femoral neck and base plate of the prosthesis can easily be appreciated.

The cup was loose enough to be removed with fingers alone. The acetabular bed was coated with fibrous tissue. When this was removed, the bony bed was noted to be sclerotic. In areas where the cup thread had been moving there were areas of necrotic bone which were discoloured. Insufficient additional bone medial to the bed was present to permit excessive further reaming and this measured only 3 mm. Osteostixis was performed to attempt to bring some vascular ingrowth to the bed but bleeding from these holes was minimal. Gentle additional reaming was performed with the 26.5 mm acetabular reamer. Small islands of cancellous bone were exposed as a result. The Kyon revision cup is a two part cup with a mesh with screw holes (for 2.4 mm diameter screws as used in the cage screws for TTA) and a titanium backed Polyethylene insert which is press fit into the mesh. This was required as a single component press fit cup could not be used due to the fact that the Helica cup plus peri prosthetic bone loss resulted in a recess greater than the diameter of the ideal cup that would be used to replace it.

The neck cut was taken down to a level that split the Third Trochanter as previously described and an extra small stem was used to permit this to be recessed below that lateral cortical defect. The collar of dense sclerotic bone associated with the track of the Helica screw thread of the stem would have required very aggressive reaming to remove with the risk of further femoral fracture and thus the extra small stem was the largest stem that would fit. Once secured by screws, stability tests could not be performed until the Greater Trochanter had been reattached. This was achieved with 14 holes of an ALPS 8 plate.

This plate was used because it is easier to contour and that mono cortical screws could be used. Screw hole 2 was deformed by contouring (due to failure to use a plug to protect the hole during contouring) and thus a standard screw was used in this position.
Following application of the plate a short head and neck was initially assessed for stability. A little laxity was noted and this was therefore changed to a medium head and neck. With this in place, all stability tests were passed showing no tendency for luxation in extension, flexion, abduction or adduction. The final stability test is to attempt to lift the patient by a retractor placed under the proximal femur. No tendency to luxate was found on this test.
A bacteriology swab was taken and closure was performed routinely.

Post Operative Radiographs

The cup position is a little more open than we would generally aim for and measures 55 degrees. The stem can be seen to be undersized but the patient body weight is 25 kg at body condition score of 3.0, the dogs anticipated body weight at ideal condition score of 2.5 would be 22 kg and thus the risk here is more than met by the need to position the stem more distally.

Risks

Luxation: This is always present with THR. The more open cup angle may increase this risk but as all stability tests were passed, this should be low.

Failure of in growth into cup: This is an increased risk due to the sclerotic nature of the acetabular bed.

Infection: As with all THR revisions, each intervention increases this risk.

Stem Fracture: This is present on the list due to the requirement to undersize the stem. It is considered a low risk based on body weight.

I will keep you posted as to how the case progresses. Feel free to comment or raise any queries regarding this case.