TPLO in toy breeds using 2.0mm and 2.4mm PAX plates

CCL rupture is common in small breeds and TPLO is an increasingly preferred method for this purpose as well as in large breeds. In the article we present to you, Dr. Sv. Hristov shares his experience in usage of this technique for toy breeds with the PAX system, provided in South Eastern Europe by VetWest.

01) TPLO in toy breeds, what do we know

  1. Small breed dogs tend to present with CCL rupture later in their life (5-10 years)
    compared to large dogs.
  2. Obesity has been suggested to be a risk factor for CCL rupture.
  3. Different morphology of the proximal tibia in small dogs – steeper TPA angle (28.0 –
    36.3 degree), absent distal flare of the tibial tuberosity and caudally bowed fibula.
  4. No clear evidence which treatment option is superior. Conservative management
    vs. Surgical treatment – what is known is that in conservative treatment the
    recovery is prolonged up to 4 months.
  5. TPLO – currently the most reported surgical treatment in small breed dogs with low
    complication rate and good subjective outcome. But compared to large dogs, the
    procedure is technically more difficult and with little room for errors due to the
    small size of the proximal tibia especially in toy breed dogs.
  6. The steeper TPA in small dogs requires larger amount of rotation of the proximal
    tibial segment often beyond the so called “safe point” which is the insertion point
    of the patellar ligament. Despite that, the risk of tibial tuberosity fracture is low, but
    the surgeon should aim to preserve a tibial tuberosity base width of 6mm when
    performs the radial osteotomy.

*Cranial cruciate ligament rupture in small dogs (<15kg): a narrative literature review. V.Brioschi, G.I. Arthurs. JSAP, 2021, 1-14

02) Short characteristics of the PAX system

Multidirectional locking system.
Up to 10 degree of drilling angle is the range which doesn’t affect the
locking mechanism and the push-out strength of the locking unit.
Screws are made of titanium alloy and are twice harder than the
titanium PAX plate. This allows structural deformation of the vertical
plate hole ridges by the sharp, cutting threads of the harder screw
head during tightening.
A guideline is provided for screw tightening of 2.0 and 2.4mm PAX
screws and the required torque value is between 0.7-0.8Nm. This
torque can be achieved using a dynamometric screw driver but in the
clinical practice it will be important that the head of the screw to be
flush with the plate surface.


  1. Proximocaudal curve to optimize
    plate alignment and screw purchase
  2. Crescent head to accommodate high
    tibial osteotomies
  3. Titanium alloy plates and screws for
    supreme biocompatibility

The effect of screw angulation and insertion torque on the push-out strength of
polyaxial locking screws and the single cycle to failure in bending of polyaxial
locking plates. B.W.Bufkin et al, VCOT, 2013

03) Personal experience with PAX 2.0 recon T-plate

5-hole 2.0 mm recon plate has been used from the author for large amount of TPLO procedures
in toy dogs before the introduction of the new pre contoured crescent head plates.
Despite that the plate fits well and it is easy to contour to the proximal tibial segment, some
disadvantages are still present. The reconstruction characteristics of the plate makes it potentially
prone for breakage. When a relatively large amount of rotation is required (which seems quite
often in toy breeds due to their higher TPA), plate is not align to the tibial axis but it is more
oblique positioned on the bone which makes the construction mechanically weaker.
On the other hand, the presence of the holes for temporary pin fixation facilitates the temporary
plate position on the bone and prevents the so called “helicopter effect” while the first screw is
No secondary loss of reduction, broken plate or broken screws are seen within our caseload.

04 Preoperative planning

*4.5 kg male Biewer Terrier, acute, complete cranial cruciate ligament rupture of his left stifle

The preoperative planning has been done using software planning tool (vPOP Pro) based on the M/L
of the intact right stifle joint.
TPA is measured and most suitable radial saw is selected. Chord length of 4.0 mm is determined
based on TPLO chart for achieving between 0 and 5 degree of postoperative TPA angle.
Different type of TPLO plates have been templated. Synthes type 2.0mm TPLO plate doesn’t seem to
fit well to the proximal tibial segment as the screws are either too close to the osteotomy or very close
to the joint surface. On the other hand, the proximal aspect of the plate obscures the joint surface and
determination of the achieved TPA. The Clover Micro TPLO plate fits well to the proximal segment but
the Micro series are designed for a dogs up to 3 kg and the decision was made not to be used in this

2.0mm crescent head TPLO PAX plate has been chosen as it fits best to the planned saw blade size and the proximal tibial segment

05 Step by step application of the plate

The radial osteotomy is executed and best position of the plate determined. A 1.1mm pilot hole is made and 1.5mm temporary cortical screw inserted in the middle screw hole of the distal part of the plate. This temporary screw seems important for
bringing the plate against the diaphysis of the tibia and for achieving plate stability while inserting the proximal locking screws.
The proximal two locking screws are inserted. The system allows combination of 1.5, 2.0 or 2.4mm locking screws. As the head
of the plate accommodates only two locking screws, the biggest diameter screws that fits to the plate are best to be used. A
locking screw in a compression mode is next inserted in the most proximal hole of the distal tibial segment. A compression
guide is used to drill eccentrically into the plate hole. Locking screw is inserted and the temporary 1.5mm cortical screw is
slightly loosed. The locking screw is then tighten which allows slight axial translation of the proximal tibial segment and
achieves osteotomy compression.

The cranial aspect of the plate is
always offset from the bone surface
The caudal aspect of the plate should
be touching the bone surface

06 Postoperative assessment

Two orthogonal radiographs are obtained.
The four “A”s principle of postoperative
assessment is used:
1)Alignment – pays attention to a torsional
or angular alignment of the two tibial
2)Apposition – loss of apposition or so
called “balcony effect” is what is required
after TPLO. An osteotomy gap has to be
avoided along the entire radial osteotomy
3)Apparatus – implant’s location, size and
position. Notice the well aligned plate over
the typical sigmoid tibia in small breeds
which often makes challenging the
position of the most distal plate screw
4)Accuracy – in small and toy breed dogs
we aim for 0 to 5 degree of TPA
postoperatively. This is a bit lower than
what is generally recommended, but
cranial cruciate deficient stifle in small
breeds tends to be more unstable
probably due to soft tissue laxity