Keywords medial patellar luxation - tibial tuberosity transposition - locking plate fixation
- unilateral surgery - single-session bilateral surgery
Introduction
Medial patellar luxation (MPL) is a common cause of pelvic limb lameness in small
breed dogs.[1 ]
[2 ]
[3 ]
[4 ] Medial patellar luxation (MPL) is usually a developmental abnormality but can also
be induced traumatically. Surgery is recommended for symptomatic dogs with MPL classified
as grade 2, 3, or 4.[1 ]
[2 ] Tibial tuberosity transposition (TTT) is a surgical procedure to realign the quadriceps
mechanism with the trochlear groove. The transposed tibial tuberosity (TT) can be
fixated with one or two pins with or without a tension band wire (TBW) with reported
11 to 43.1% overall complication rate.[3 ]
[5 ]
[6 ]
[7 ]
[8 ]
[9 ] It would be desirable to have a reliable TT fixation method with a lower complication
rate. Alternative TT fixation methods have been developed including a pin or screw
placed medial to the transposed TT,[10 ]
[11 ] an “impact plate” and a curved plate with locking screws,[12 ] a trapezoid plate and nonlocking screws,[13 ] and a hemicerclage.[14 ]
Locking plates and screws are commonly used for fracture or osteotomy stable fixation.[15 ]
[16 ] The use of a locking plate and pin TT fixation (Plate–Pin) in a cadaveric in vitro
mechanical study was reported showing a higher load at failure values compared with
Pin–TBW fixation.[17 ] Mechanical testing does not replicate physiologic loading or cyclic fatigue encountered
during the postoperative convalescent period. Hence testing with clinical cases is
needed to evaluate different methods of TT fixation. The use of locking plates has
not been reported for TT fixation during MPL surgery in dogs or cats.
The objectives of this study were to assess outcome and complication rate in dogs
with MPL undergoing TTT surgery using a Plate–Pin fixation.
Materials and Methods
Case Selection
This multicenter study included all dogs with grade 2 to 4 MPL treated surgically
with TTT Plate–Pin fixation between December 2019 and May 2023. The operations were
performed in three veterinary hospitals by three surgeons: Esa Eskelinen (n = 26), Ari Suhonen (n =21), and Johanna Makitaipale (n = 18) MPL Plate–Pin procedures. The surgeons were nonboarded with over 30 years,
over 10 years, and over 15 years of experience, respectively. Anesthesia, surgical
technique, postoperative care, and follow-up examination were described in written
format to maintain consistency among surgeons and centers and agreed upon by participating
surgeons.
The inclusion and exclusion criteria are presented in [Supplementary Appendix Table A1 ] [available in the online version].[18 ]
[19 ]
[20 ] The femoral neck anteversion angle and the depth of the patellar groove were not
considered relevant as inclusion or exclusion criteria. The owners were preoperatively
informed of the results of the cadaveric study of the Plate–Pin fixation[17 ] and the experimental nature of the use of the locking plate fixation in clinical
cases.[21 ] If an owner did not want to participate in Plate–Pin study, two pins and a TBW were
used for TT fixation.
Perioperative Care
All dogs included in the study received perioperative cephazolin as per inclusion
criteria.[22 ] The first dose was given 30 minutes before the skin incision, at 90-minute intervals,
and was discontinued immediately after surgery. All dogs were given bupivacaine (1 mg/kg)
and morphine (0.1 mg/kg) epidurally. Either carprofen (2.0 mg/kg twice daily [b.i.d.]
orally [p.o.]) or meloxicam (0.1 mg/kg once daily p.o.) was given for 14 to 21 days
postoperatively. Anesthesia and perioperative medications are described in detail
in Appendix 1 [online only].
All dogs were discharged from the hospital on the same day within 2 to 8 hours after
surgery. At discharge, owners were all given the same written instructions for postoperative
care including medications, wound care, possible complications, and a basic at-home
rehabilitation program.[23 ] On-leash-only walks for 6 weeks on a good traction surface was recommended. Vigorous
exercise, slippery surfaces, and stairs were to be completely avoided for 6 weeks.
Trazadone (10 mg/kg b.i.d.) was dispensed on an as needed basis to help control activity
at home for 6 weeks. Owners were instructed to return for a follow-up examination
and radiographs 10 weeks after surgery. Normal activity was allowed after the follow-up
examination with radiographic evidence of bone healing and stability of the transposed
TT.
Surgery
All Plate–Pin procedures included a parapatellar approach to the stifle starting with
a craniolateral skin incision. The incision extended distal to the femorotibial joint
on the medial side to expose the proximal medial tibia. The tibialis cranialis muscle
was left intact on the lateral aspect of the tibia. The joint was opened through a
lateral retinaculum incision and the patella was luxated medially. The cranial cruciate
ligament and menisci were evaluated. A trochlear wedge recession[1 ] was performed if the femoral trochlear groove was subjectively considered to be
of inadequate depth. Templates were preoperatively superimposed on the mediolateral
radiographs to select the optimum plate size, and sizing was confirmed during surgery
with the plate placed on the tibia ([Fig. 1 ]). The largest plate that fitted perfectly on the TT and tibial diaphysis was chosen.
The proximal end of the osteotomy was located cranial to the long digital extensor
tendon groove. The distal end of the osteotomy was in the mid-section of the plate
that is void of screw holes. At the distal end of the osteotomy, the craniocaudal
width of the osteotomized TT was less than one-third of the entire craniocaudal dimension
of the tibia to minimize the risk of tibial diaphyseal fracture.[24 ] The desired and intended TT osteotomy line was marked with electrocautery with the
plate laying in its intended location. Optionally, the distal end of the TT osteotomy
was marked with a 1.1 to 1.5 mm diameter drill hole. The markers helped to identify
and localize the distal end point of the TT osteotomy and to create a precise fit
for the plate. The TT osteotomy was made using an oscillating saw perpendicular to
the sagittal plane of the proximal tibia. With the osteotomy completed, the TT was
transposed carefully and slowly. If only a small amount of transposition was needed,
the bone at the distal end of the TT osteotomy was bent during TT positioning. If
more transposition was needed, a hinged osteotomy was created at the distal end of
the TT osteotomy. It was at surgeon's discretion to drill two or three 1.1 to 1.5 mm
diameter partial or full thickness holes on the medial aspect of the tibia at the
distal end of the intended TT osteotomy ([Fig. 1B ]). This facilitated the hinged fracture to occur at the intended location at the
distal end of the TT osteotomy and minimized the risk of an inadvertent fracture of
the more proximal TT during transposition.
Fig. 1 The medial aspect of the right proximal tibia of a 7-year-old 5.7 kg Miniature Poodle
with a grade 2 medial patellar luxation (A ). The insertion of the patellar ligament is shown (white arrow). The distal end of
the osteotomy is shown (white dot). A 2.0/1.5 mm locking plate is in place in its
intended location. The tibial tuberosity transposition osteotomy line was marked with
monopolar electrocautery (white arrowhead) on the tibia. Optionally, a 1.5 mm diameter
hole can be drilled at the distal end of the intended osteotomy. It was at surgeon's
discretion to drill two or three 1.1–1.5 mm diameter partial or full thickness holes
(B , white circles) on the medial aspect of the tibia at the distal end of the intended
TT osteotomy. The distance from the osteotomy to the caudal cortex of the tibia (black
double arrow) should be as wide as possible to minimize the risk of tibial diaphyseal
fracture. The distance from the osteotomy to the cranial cortex of the tibia is shown
(white double arrow).
The TT was initially stabilized with a single pin ([Fig. 2A–C ]). Patellar alignment in the trochlear groove was then evaluated. If necessary, the
pin was removed, and the TT was repositioned into a more perfect alignment. The transposed
TT was cranially-caudally compressed against the tibia with pointed bone holding reduction
forceps to close the gap between the osteotomy and the proximal tibia. The straight
locking plate (Veterinary Orthopedics Scandinavia Ltd, Helsinki, Finland) was contoured
in the sagittal plane and mediolaterally using plate bending pliers to match to the
medial surface of the TT and proximal tibia. The most proximal screw (Veterinary Orthopedics
Scandinavia Ltd, Helsinki, Finland) was applied first. The most distal screw was applied
second taking care to position it near the center of the tibial diaphysis. Locking
screws were used. The screws were bicortical except the most distal screw, which was
monocortical. The pins, plates, and locking screws were 316L stainless steel alloy.
Capsulorrhaphy was performed along the retinacular incision to imbricate the joint
capsule and lateral parapatellar fascia as subjectively deemed necessary. The joint
capsule and deep fascia were closed with monofilament cruciate pattern sutures of
absorbable polydioxanone (MonoPlus, B. Braun AG, Melsungen, Germany). Subcutaneous
closure was a continuous pattern with the same material or with absorbable monofilament
suture (Monosyn, B. Braun AG, Melsungen, Germany). The skin was closed with monofilament
antibacterial absorbable sutures (Monocryl Plus 4–0, Ethicon, Johnson & Johnson, New
Brunswick, New Jersey, United States), in a continuous intracutaneous suture pattern.
A sterile adhesive bandage was applied over the wound. Mediolateral and craniocaudal
radiographs were acquired to document the Plate–Pin fixation construct.
Fig. 2 Plastic bone model after tibial tuberosity transposition. Tibial tuberosity fixation
was with a locking plate and a pin (black arrow) (A medial view, B cranial view, C lateral view). An alternative positioning of the pin is shown by a black arrowhead
and black line (A– C). The cranial aspect of the long digital extensor tendon groove
is shown (C, black asterisk). The most distal locking screw through the plate is monocortical
and should be positioned near the center of the tibial diaphysis (A, white arrow)
to minimize fracture at the stress riser.
Complications
Minor complications were defined as those that resolved without surgery including
seroma, superficial incisional surgical site infection (SSI), pin breakage, or pin
migration. Major complications were defined as those that required surgical intervention,
including deep incisional SSI and pin migration with seroma requiring pin removal.[25 ] Tibial tuberosity fixation failures were defined as those major complications in
which the TT fractured or avulsed. Surgical site infections were defined based on
the criteria adapted from the Centers for Disease Control and Prevention.[26 ]
Follow-Up
Outcome and complications were assessed during the follow-up examinations. Lameness
was assessed by observation during ambulation and by questioning the owner if there
were gait abnormalities observed or if the gait had returned to normal. Lameness was
classified into five grades ([Supplementary Appendix Table A2 ] [available in the online version]).[27 ] Patellar alignment was assessed during physical examination with no analgesia or
sedation and was classified into four grades.[28 ] Craniocaudal and mediolateral radiographs were acquired to evaluate implant fixation,
stability, and osteotomy healing. The radiographs were compared with the immediate
postoperative radiographs to evaluate bone healing, integrity of the implants, and
TT position.
Mid-term or long-term[25 ] outcome was assessed using the ACVS COI (American College of Veterinary Surgeons
Canine Orthopedic Index) questionnaire,[19 ] which was translated to the client's native language. Each question was answered
with points awarded 0 to 4. The best possible outcome score of 16 questions was 0
points and worst 64 points. The owners were also asked if they observed any signs
of SSI.
Statistical Analysis
Statistical analyses were performed to identify variables associated with increased
complication risk. Due to the small number of observations across levels of complication
severity, complications were coded as a simple occurrence/nonoccurrence. Separate
exact logistic regression[29 ] models were estimated for each of eight predictors using R statistical software[30 ] and the “elrm” package.[31 ] Due to their rare occurrence in the sample, surgical errors and other surgical deviations
were combined into a single variable. One control variable was selected for each based
on veterinary orthopedic knowledge and statistical comparisons ([Supplementary Appendix 2 ] and [Table A9 ] [available in the online version]). The level of statistical significance was set
at p <0.05 for two-sided analyses.
Results
Patient Characteristics
All patellar luxation cases were developmental with none being traumatic in origin.
At the first presentation, the ages ranged from 9 to 125 months (median: 32 months)
and body weight was 2.1 to 15.7 kg (median: 6.2 kg). Additional patient characteristics
are presented in [Supplementary Appendix Tables A2 ] and [A4 ] (available in the online version).
Surgical Procedures
Details of the surgical procedures are presented in [Supplementary Appendix Table A4 ] (available in the online version). The diameter of the pins used in this study ranged
from 0.9 to 1.4 mm. The locking plates were 1.5 mm (TT)/1.5 mm (diaphysis of tibia),
1.5 mm/2.0 mm, and 2.0 mm/2.4 mm ([Supplementary Appendix Table A5 ] [available in the online version]). The sizes of the implants were selected based
on the size and conformation of the tibia, not the weight of the dog.
Follow-Up
In-clinic follow-up examination and radiography was performed at a median of 68 days
(range: 33–648 days) postoperatively ([Supplementary Appendix Fig. A1 ] and [Supplementary Appendix Table A6 ] [available in the online version]). In two stifles, a tibial shaft fracture ([Fig. 3 ]) and a TT fracture ([Fig. 4 ]), the follow-up was performed after the fracture repairs. Lameness resolved in all
stifles and patellar luxation in 64/65 stifles. Grade 1 MPL was detected in one stifle.
Lameness and grade of patellar luxation are in [Supplementary Appendix Table A2 ] (available in the online version). Orthogonal radiographs confirmed bone bridging
of the osteotomy and unchanged position of the TT compared with immediate postoperative
radiographs in all stifles. Implants were stable and intact in 57/65 stifles. Minor
complications were reported in 8/65 and major complications in 6/65 stifles, adding
up to a total of 14/65 stifles with complications. Superficial incisional SSIs were
reported in 3/65 stifles.
Fig. 3 A medial-lateral radiograph (A ) of the stifle and fractured left tibia of an 8.3 kg German Spitz Mittel, 5 days
after TTT surgery after jumping and falling. TT fixation was with a locking plate,
1.5 and 2.0 mm locking screws, and a 1.0 mm pin. The most distal screw was placed
in the caudal cortex of the tibia, which predisposed to the fracture. A cranial-caudal
radiograph (B ) of the same fracture. A medial-lateral radiograph (C ) and cranial-caudal radiograph (D ) of the stifle and fractured left tibia 8 weeks after fracture fixation with a 2.7 mm
locking plate and screws.
Fig. 4 A medial-lateral radiograph (A ) of the stifle and left tibia of an 11.2 kg mixed breed dog 8 weeks after TTT surgery
with a locking plate, 2.0 and 2.4 mm locking screws, and a 1.2 mm pin. A fracture
of the TT occurred when the TT was transposed during the surgery. The most proximal
fragment of the TT displaced after surgery causing patella alta . A medial-lateral radiograph (B ) of the stifle 8 weeks after the TT fracture fixation with a locking plate, 2.0 and
2.4 mm locking screws, a 1.2 mm pin, 0.9 mm cerclage wire, and a patellar ligament-circumpatellar
loop with 5 metric polyamide suture (Dafilon, B. Braun AG, Melsungen, Germany). The
plate has two 1.0 mm holes at the proximal end and one in the middle of the plate.
The cerclage wire was anchored through the 1.0 mm holes. In index surgery the plate
was shortened through the proximal 1.0 mm holes for better fit.
In 56/65 procedures there were no deviations from the surgical protocol or implant
positioning, described in the Surgery section. Minor complications of the surgeries
were: seroma at the cranial end of the pin (n = 1), pin bending (n = 1), pin breakage (n = 2), screw breakage (n = 3), and superficial incisional SSI (n = 2). One stifle had two minor complications: screw breakage and superficial SSI.
One stifle had three minor complications: pin bending, screw breakage, and superficial
SSI. Major complications were pin migration (n = 1) and capsulorrhaphy failure (n = 1) in which there was dehiscence of the lateral joint capsule and fascia closure.
In 9/65 procedures, there were deviations from the surgical protocol or implant positioning.
The procedures with deviations and their outcomes are presented in [Supplementary Appendix Table A7 ] (available in the online version). Surgical deviations were found to significantly
increase the risk of postoperative complications, odds ratio (OR) = 11.3, p < 0.05 ([Supplementary Appendix Table A8 ] [available in the online version]). Based on additional exploratory analyses (Appendix
2 [online only]), it is likely that surgical errors contributed considerably more
to this effect than did other surgical deviations. The effects of all other predictors
were nonsignificant.
The COI questionnaire was acquired at a median of 14 months (range: 6–39 months) postoperatively
as detailed in [Supplementary Appendix Table A6 ] (available in the online version). The median COI score was 3 (range: 0–21; [Supplementary Appendix Fig. A2 ] [available in the online version]). One case, a single-session bilateral surgery,
had a COI score of 21 for both the left and the right stifle. This dog scored 4 for
the question “Jumping up (as in getting into the car or onto the bed).” This 2.1 kg
Chihuahua has never been able to jump into the car or onto the bed. The score for
question “How often did your dog “pay” for over-activity, with increased pain or stiffness
the following day?” was 3. The scores for other COI questions of this dog were 0 to
2.
Discussion
Complications of MPL TTT surgery frequently require additional surgery,[5 ] and dogs with complications are more likely to have a poor outcome.[32 ] The complication rates of the Plate–Pin fixation and comparison with other reported
TT fixation methods are presented in [Supplementary Appendix Table A9 ] (available in the online version). Direct comparison between different studies is
difficult because of different inclusion criteria, different follow-up time frames,
potential selection bias and complications classified differently.[1 ]
[5 ]
The incidence of postoperative MPL recurrence in Pin–TBW TTT surgeries ranges from
5 to 12.4% and is reported to be more prevalent in large than small breed dogs.[5 ]
[6 ]
[7 ]
[8 ]
[9 ]
[33 ] Insufficient or excessive TT transposition can predispose to luxation recurrence.
Transposition distance is based on subjective clinical judgement which is subject
to error. Tibial tuberosity fixation failure is another cause of luxation recurrence.
In our study, there were no luxation recurrences in 64/65 stifles ([Supplementary Appendix Table A9 ] [available in the online version]). This indicates correct alignment of the patella
during surgery and that the TT did not displace after surgery in 64/65 stifles. With
Plate–Pin TT fixation, the surgeon can achieve marked transposition of the TT, which
is necessary in grade 3 and grade 4 MPL surgery.
Bilateral MPL is sometimes treated during single-session bilateral surgery for economic
reasons.[7 ] Recent studies comparing complication rates in dogs undergoing single-session bilateral
MPL surgery and unilateral surgery have shown contradictory results.[7 ]
[34 ]
[35 ]
[36 ] In our study, complications occurred in 11/55 unilateral and in 3/20 single session
bilateral stifle surgeries.
The incidence of Surgical site infections in dog and cat surgical procedures classified
as clean is 2.0 to 4.9%.[37 ]
[38 ] Plate–Pin and TPLO surgery are classified as clean orthopedic procedures in which
a locking plate is applied to the proximal tibia. Surgical site infections develop
in 8.4 to 11% of dogs following TPLO surgery.[39 ]
[40 ] In our study, SSIs occurred in 3/65 stifles (4.6%).
Implant-associated complications are the most common reported complications following
MPL surgery.[1 ] Pins can cause complications when they migrate or break.[1 ]
[3 ]
[5 ]
[6 ]
[7 ]
[8 ]
[9 ] In our study, most of the complications (8/65 stifles, 8/20 complications) were
problems associated with the pin. We subjectively chose a smaller pin size for Plate–Pin
fixation than would be chosen on a Pin–TBW TTT surgery. Despite the small pin size
and pin-related complications, all TT osteotomies healed and the TT fixation with
the locking plate did not fail in cases performed as described in the Surgery section.
It remains equivocal if the pin adds any advantages in TTT surgery with locking plate
fixation. A pin could be used only for temporary fixation to hold the TT during patella
alignment, before locking plate application ([Fig. 2 ], arrowhead),[10 ] and then be removed. Alternatively, instead of using a pin for permanent fixation,
a cortical screw[10 ] or an “impact plate”[12 ] could be placed adjacent to the TT.
In statistical analysis, a lower dog weight was significantly associated with an increased
risk of complications in the baseline model (OR = 0.768, p < 0.05), wherein a 1 kg increase in weight implied a 23.2% decrease in complication
risk. After controlling for surgical deviations, however, this association became
nonsignificant. This suggests that the impact of weight on complication risk might
be partially mediated through increased incidence of surgical deviations: smaller
screws are used on smaller dogs and smaller screws also break more easily ([Supplementary Appendix Table A8 ] [available in the online version]). In 3/65 stifles (3/20 complications), one of
the two 1.5 mm screws holding the transposed TT were broken at follow-up. This indicates
that at least three screws are needed for a more secure TT fixation. Further development
of the locking plate TT fixation is needed to overcome the pin-related complications
and breakage of the TT fixation screws.
Our study has limitations. It was retrospective, there was no control group, and no
randomization.[41 ] A significant limitation regarding the statistical analyses of complication risk
was small sample size, which resulted in estimates with poor stability and limited
statistical power to reveal small to moderate effects. Consequently, the results should
be interpreted with caution. The outcome measures including lameness, degree of luxation,
assessment of healing of the TT osteotomy, and COI questionnaire answers were subjective,
which is an inherent problem with this kind of study. These factors could have caused
a variation in examination and documentation reporting among authors as has been reported
elsewhere.[11 ] The follow-up period varied and COI questionnaires from the preoperative period
were not available. Mid- or long-term outcome was assessed in most cases with the
COI questionnaire only ([Supplementary Appendix Table A6 ] [available in the online version]). Regardless, it is improbable that the TT fixation
would fail if bone bridging of the osteotomy is confirmed radiographically after surgery.
The Plate–Pin is a promising new fixation technique for TTT that is suitable for patients
with MPL. There were 9/65 stifle surgeries with deviations from the surgical protocol
with increased complication rate ([Supplementary Appendix Tables A7 ]–[A9 ] [available in the online version]). We refined the surgical technique after every
complication we encountered and we described the procedure in the Surgery section
with special emphasis on preventing the complications.[21 ] It does not mean that complications or surgical errors will be completely eliminated
in the future by every surgeon including the authors. Also new types of complications
that were not seen in our study may occur.
There were no TT avulsions, fractures, or luxation recurrences in 56/65 stifles where
the Plate–Pin surgery was performed as described. This indicates that the fixation
method could prevent complications and therefore, significant postoperative morbidity
and costs. A prospective, randomized, and preferably multicenter study is needed to
confirm if the complication rate of Plate–Pin TT fixation is significantly lower than
the complication rate of the traditional TT fixation with pins and TBW or with other
fixation techniques.[10 ]
[11 ]
[12 ]
[13 ]
[14 ]
