Patient-specific cutting blocks to improve tibial tubercle transfer surgery for patients with patellar instability

Patellofemoral instability is one of the most frequent types of knee problems
in adolescents and young adults. During a patellar dislocation the patella
dislocates from its normal position, which occurs mainly when the knee is
flexed between 0 and 30 degrees1*3. The incidence of primary patella
dislocations depends on age and varies from 31:100.000 (10-19 years), to
11:100.000 (20-29 years), and 2:100.000 (30-49 years)4*8.

Recurrent patellar instability has many causes. One of the most common causes
is a malposition of the tibial tubercle -the insertion point of the patellar
tendon on the tibia- compared to the trochlear groove. Surgical treatment aims
to restore normal patellar tracking by repositioning of the tibial tubercle in
distal and/or medial direction.The current surgical procedure is performed free
hand, completely relying on the skill and experience of the orthopaedic
surgeon.

During surgery, the tibial tubercle is detached from the tibia using a flat- or
V-shaped osteotomy. The tubercle is then re-attached at the desired position
with two countersink screws.

As a result of free hand surgery, it is hard to reproduce the pre-operatively
planned displacement. This causes inaccurate repositioning of the tubercle and
imperfect re-alignment. Imperfect realignment is associated with patient
dissatisfaction, continued instability and dislocation in 10% of the cases as
well as an increased risk of postoperative fractures 9,10.

We hypothesize that improving the accuracy of the tubercle transfer with the
aid of surgical cutting guides has a positive effect on stabilization,
postoperative fractures and pain.
The Orthopedics department of the Radboudumc has therefore developed
patient-specific surgical cutting guides for tubercle transfer surgery.

The primary goal of the current research is to determine the accuracy of the
procedure using patient-specific cutting guide. Secondary endpoints are
surgical time needed and the guides* fit onto the patients* tibia.

Single institution, prospective, interventional study in the Netherlands.
We will include 10 patients with recurrent patellofemoral instability, for whom
conservative treatment has failed and tubercle transfers surgery is indicated.
Outpatients will be recruited by Dr. Ing. S. van de Groes, orthopedic surgeon
in Radboudumc.

In the current standard clinical practice, a static CT scan and X-ray are made
to investigate the tibial tubercle-trochlear groove (TTTG), height of the
patella and the shape of the trochlea. Based on the scans a surgical treatment
plan is established. During the follow-up after the surgery an X-ray is made.

Based on the preoperative CT scan and the preoperative planning made by the
orthopedic surgeon, patient-specific cutting guides will be developed. The
cutting guides have already been thoroughly tested in cadaver studies.

In this study, the preoperative CT scan will be used to create patient-specific
cutting blocks to transfer the tibial tubercle to a new location, which is
determined by orthopedic surgeon Dr. Ing. S. van de Groes. A difference in
comparison to the standard clinical practice is that patients will get a
postoperative CT scan, instead of the X-ray. This CT scan is made the day after
surgery, when the patient is still in clinic.

The postoperative CT scan will be used to determine the position of the
tubercle after surgery. By measuring the difference between the postoperative
and planned position, the agreement between the surgical planning and surgery
can be established.

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The conventional postoperative X-ray will be replaced by a CT scan for patients
included in the study. A CT scan is associated with an increased dose of
ionising radiation. A normal X-ray is associated with 0,6 µSv, a CT scan with
0.020 mSv, the increase in dose is trivial compared to natural background
radiation (2mSv in the Netherlands).