Keywords
medial patellar luxation - tibial tuberosity transposition - trochlear block recession
- trochlear wedge recession - tension band wire
Introduction
Medial patellar luxation is a common cause of pelvic limb lameness in dogs, particularly
in small breeds, with 52 to 65% being affected bilaterally.[1]
[2]
[3]
[4]
[5] Although the exact cause of patellar luxation is unclear, this condition is considered
to be developmental, resulting from multiple anatomical abnormalities of the pelvic
limbs.[4]
[5]
[6]
[7]
[8] Patients with clinical signs associated with medial patellar luxation can be treated
surgically with repair techniques that are specifically indicated in cases classified
as grades II to IV.[9] The aim of surgery is to realign the stifle extensor (quadriceps) mechanisms and
to reposition the patella within the trochlear groove. Corrective surgical techniques
have been investigated and the outcomes have been reported in several studies.[8]
[9]
[10] The most common surgical technique to realign the quadriceps mechanism is a tibial
tuberosity transposition (TTT), which is often combined with a femoral trochleoplasty,
lateral soft tissue imbrication and medial soft tissue release.[10] However, there are individual variations in these techniques, such as the way in
which the trochleoplasty is performed and the way in which the tibial osteotomy is
stabilized.
The overall complication rate following stabilization of unilateral patellar luxation
has been reported as 13 to 45% including all the surgical correction for different
medial patellar luxation grades.[1]
[8]
[9]
[10]
[11]
[12]
[13]
Complications include patellar reluxation, implant failure or migration, tibial tuberosity
fracture and tibial tuberosity avulsion, recession wedge displacement, tibial or femoral
fracture, lateral trochlear ridge fracture, patellar ligament rupture, inability to
fully extend the stifle joint, osteomyelitis, wound dehiscence and septic arthritis.[1]
[8]
[9]
[10]
[11]
[12]
[13]
The primary aim of this study was to determine if the method of trochlear recession
(block vs. wedge) and the implant construct (1 pin vs. 2 pin and tension band wire
[TBW] vs. no TBW) used to stabilize the TTT influenced the complication rate of patients
weighing <20 kg treated for medial patellar luxation. In light of our clinical experience,
we hypothesized that the type of surgical technique performed in dogs weighing < 20 kg
would not affect the incidence of complications. Another aim was to report short-
and long-term complications (>12 months) for our population and to determine risk
factors associated with these complications.
Materials and Methods
This retrospective observational study used anonymized clinical data and was approved
by the Liverpool Veterinary School Research Ethics Committee (VREC470). Patient records
were searched to identify dogs (< 20 kg) that underwent surgery for medial patellar
luxation between 2011 and 2016.
Dogs were excluded from the study if they had a suspected traumatic patellar luxation,
had concurrent orthopaedic or neurological abnormalities (such as cranial cruciate
ligament rupture), had previous surgery on either pelvic limb, or if the surgical
stabilization was performed with techniques other than trochleoplasty, TTT and soft
tissue imbrication or release. Dogs that were re-presented during the study period
for treatment of a contralateral medial patellar luxation were included in the study.
Information retrieved from the records included signalment, orthopaedic examination
findings, pre- and postoperative radiographic or computed tomographic findings, type
of surgical treatment, surgical and anaesthetic time, intraoperative and postoperative
complications and need for further surgical or medical treatment. The grade of patellar
luxation was recorded according to the Singleton grading system.[14] Patients were re-examined ∼8 weeks following surgery to evaluate clinical progression
and repeat orthogonal radiographs. Patients that exhibited clinical deterioration
in the first 8 weeks following surgery were examined sooner and were also examined
at any point afterwards if there was any deterioration. Long-term follow-up to detect
unreported complications was evaluated using the referring veterinarian clinical notes
with a minimum postsurgical time of 12 months.
Surgical Treatment
Technique combinations performed to achieve intraoperative stability of the patella
included medial soft tissue release (medial capsule and retinaculum), lateral capsule
and retinaculum imbrication, trochleoplasty and TTT.
All surgical procedures included a lateral parapatellar approach to the stifle joint
and an arthrotomy to evaluate the femoral trochlear groove, patellar articular cartilage,
cruciate ligaments and menisci. If the femoral trochlea was subjectively considered
to be of inadequate depth, a trochlear wedge recession[15] or a trochlear block recession technique[16] was performed based on surgeon preference. A TTT was performed if the tibial tuberosity
was found to be medially translated, transposing the tibial tuberosity laterally until
the quadriceps mechanism was subjectively realigned and the patella could no longer
be luxated. The TTT was then stabilized with one or two Kirschner wires (K-wire) with
or without a TBW. The K-wires ranged from 0.9 to 2.0 mm, were usually unthreaded and
had a trocar point in each end. The point of insertion of the K-wire was recorded
as proximal to the tibial tuberosity, at the level of the tibial tuberosity (patellar
ligament insertion area) or distal to the tibial tuberosity (or at the tibial crest).
When two K-wires were placed, the insertion point was recorded for the most proximal
pin, and their location, relative to each other, was described as being vertical or
horizontal ([Fig. 1]), as previously described.[9] The direction of the K-wire was also recorded as caudoproximal, caudodistal or perpendicular
to the anatomical axis of the tibia. Soft tissue balancing procedures, including medial
release and lateral imbrication, were performed as deemed necessary by the surgeon
to ensure normal tracking and stability of the patella throughout stifle range of
motion. Orthogonal radiographs were obtained immediately after surgery to assess implant
positioning.
Fig. 1 Mediolateral and caudocranial postoperative radiographs of the stifle showing the
position of the two Kirschner wires, relative to the most proximal one, as (A and B) vertical or (C and D) horizontal.
Complications
Orthopaedic and radiographic assessment was completed at 6 to 8 weeks post-operatively
or sooner if any deterioration occurred. When a complication occurred, the patient
was treated medically or surgically, having subsequent periodic follow-up appointments
until the complication was considered resolved. Complications were standardized as
previously defined.[17] To detect any unreported short- or long-term complications associated with the surgery,
the referring veterinarians were contacted, and the complete clinical notes of each
patient were reviewed.
Statistical Analysis
Sample Size Estimation
A power calculation was used to determine the number of surgeries required to detect
a difference in complication rate amongst different surgical procedures. Assuming
an overall complication rate of 40%,[9]
[12] and an α of 0.05, it was estimated that a total of 98 surgeries would be required
for an 80% power to demonstrate a 25% difference (i.e. 25% difference between the
means of the groups) in complication rate between methods.
Initial Statistical Comparisons
Initially, Fisher's exact test was used to compare differences in the number of complications
with different case presentations and surgical methods. Presence or absence of a complication
was the dependent variable. Continuous explanatory variables assessed included were
age, weight, anaesthetic time and surgical time. Using a Shapiro–Wilk test, none of
these datasets were likely to be from a normal distribution (p < 0.001 for all) and, therefore, they are reported as median (range). Categorical
variables assessed were sex, neuter status, breed, pre-surgical medial patellar luxation
grade, use of trochleoplasty, type of trochleoplasty (trochlear wedge recession vs
trochlear block recession), use of medial release, use of lateral imbrication, use
of TBW, number of K-wires, K-wire point of insertion, K-wire direction, TTT distal
cortical attachment, surgeon performing the procedure (senior surgeon vs. resident
under supervision), K-wire alignment (only when two pins were used) and the use of
postoperative antibacterial therapy. With regard to the four different constructs
(1 pin, 1 pin + TBW, 2 pins, 2 pins + TBW), Fisher's exact test was initially used.
Since there was no difference between the four groups, another analysis was performed
taking into account the single variable (number of pins and +/− use of TBW). Finally,
the Fisher's exact test was used to determine whether TBW was more likely to be used
in cases without an intact distal tibial tuberosity attachment; further, the bodyweight
of dogs that did or did not have a TBW was compared with determine whether the use
of this procedure differed according to dog size.
Determining Factors Associated with the Risk of Complications
Simple and multiple logistic regression were used to determine the association of
a range of variables with the occurrence of complications. Initially, the outcome
variable was occurrence of any type of complication, and the explanatory variables
were those listed above for Fisher's exact test. These variables were first tested
separately with simple logistic regression. In addition to assessing both bodyweight
and the use of TBW separately in simple logistic regression, the interaction between
these factors was also tested. Similarly, the interaction between use of a TBW and
whether or not the distal tuberosity attachment remained intact was also tested. A
multiple logistic model was then built, which initially included the variables identified
as p < 0.2 by simple regression. Contingency tables were used to assess the independence
of each factor vis-à-vis other factors. The model was then refined over multiple rounds,
using backward-stepwise elimination of the least significant variable each time, and
variables were only retained in the final model if they were significant in their
own right (p < 0.05). To characterize further the variables associated with complications, multiple
regression analysis was repeated separately for different types of complications:
tibial tuberosity fracture or avulsions, patella reluxation and implant-related complications
(including seroma, skin irritation or any cause of implant removal, e.g. pin migration
or patella tendinosis). Logistic regression results are reported as odds ratios, 95%
confidence intervals and the associated p-value. The level of statistical significance was set at p < 0.05 for two-sided analyses.
Results
In total, 87 dogs met the eligibility criteria for the study and a total of 100 surgical
procedures for medial patellar luxation were performed. There were 35 male (23 neutered)
and 52 female dogs (34 neutered). At the time of the surgery, the median age was 20
months (range, 3–96 months) and median weight was 8.2 kg (range, 1.2–20.0 kg). The
most common represented breeds included Cavalier King Charles Spaniel (n = 17), Chihuahua (n = 9), Yorkshire Terrier (n = 9), French Bulldog (n = 5) Jack Russell Terrier (n = 4), Bichon Frise (n = 3), Border Collie (n = 3), Griffon Bruxellois (n = 3), Pomeranian (n = 3), Staffordshire Bull Terrier (n = 3) and Toy Poodle (n = 3). Preoperative imaging aimed to characterize the deformities present in each
individual case, and consisted of orthogonal radiographs of femurs, tibias and stifles
or computed tomography of both pelvic limbs, or a combination of both imaging modalities.
In our study population, corrective osteotomy techniques of the distal femur or proximal
tibia were not deemed necessary based on these preoperative imaging studies. Surgery
was performed on the left stifle in 60 cases and on the right stifle in 40 cases.
The luxation grade was classified as grade I in 1 stifle, grade II in 51 stifles,
grade III in 42 stifles and grade IV in 6 stifles.
Surgical Procedures
Median surgical time was 75 minutes (range, 25–195 minutes) and median anaesthetic
time was 135 minutes (range, 50–270 minutes). Surgery was performed by an experienced
surgeon in 70 cases and by a resident under supervision in 30 cases. Thirteen dogs
had bilateral medial patellar luxation surgery at least 8 weeks apart. Femoral trochleoplasty
was performed on 90 stifles and included a trochlear wedge recession in 68 stifles
and a trochlear block recession in 22 stifles. Medial soft tissue release was performed
on 41 stifles and lateral imbrication was performed in 81 stifles. All surgical procedures
included a lateral TTT. The tibial tuberosity was stabilized with one K-wire (range,
0.9–1.8 mm) in 28 stifles, one K-wire (range, 0.9–1.6 mm) and TBW (range, 0.8–1.1
mm) in 26 stifles, two K-wires (range, 1.1–1.6 mm) in 20 stifles and two K-wires (range,
0.9–1.6 mm) and TBW (range, 0.8–1.6 mm) in 26 stifles ([Fig. 2]). There was no difference in the bodyweight of dogs that had a TBW compared with
those that did not (p = 0.074).
Fig. 2 Mediolateral postoperative radiographs of the stifle showing the four different techniques:
(A) one K-wire, (B) one K-wire with tension band wire (TBW), (C) two K-wires and (D) two K-wires with TBW.
The insertion of the K-wire was proximal to the tibial tuberosity in 20 cases (20%),
at the level of the tibial tuberosity in 58 cases (58%), distal to the tibial tuberosity
in 18 cases (18%) and not assessable in 4 cases (4%). The direction of the pins was
caudodistal in 24 cases (24%), caudoproximal in 17 cases (17%) and perpendicular to
the long axis of the tibia in the remaining 59 cases (59%) ([Fig. 3]). When two K-wires were used (46 cases), the alignment was vertical in 34 cases
and horizontal in 12 cases ([Fig. 1]). On the postoperative radiographs, the distal cortical attachment of the tibial
tuberosity was deemed to be preserved in 30 stifles, not maintained in 43 stifles
and could not be assessed in 27 stifles due to rotation. There was no difference in
use of TBWs for cases where the distal attachment of the tibial tuberosity was or
was not left intact (p = 1.000). Antibacterial therapy was prescribed postoperatively at the discretion
of the surgeon, and 16 stifles received antibiotic treatment for 5 to 7 days postoperatively.
Fig. 3 Mediolateral postoperative radiographs of the stifle showing the direction of the
K-wire in relation to the anatomical axis of the tibia: (A) caudodistal, (B) caudoproximal (C) and perpendicular.
Complications
Thirty-seven stifle joints developed postoperative complications; 12 were considered
minor and 25 were considered major. No catastrophic complications were reported ([Table 1]). Twenty-eight complications (12 minor and 18 major) occurred in the perioperative
period and 7 (7 major) occurred between 3 and 6 months after surgery. No mid-term
or long-term complications were reported. Postoperative follow-up ranged from a minimum
of 15 months to a maximum of 61 months (median 27 months).
Table 1
Summary of postoperative major and minor complications following corrective surgery
for medial patellar luxation in dogs
|
Major complications
|
|
Group
|
Implant related[a]
|
Reluxation
|
TTF/Avulsion
|
Others
|
Total
|
|
1 K-wire
|
4
|
|
1
|
1 (SSI)
|
6/28
|
|
1 K-wire + TBW
|
5
|
1
|
2
|
1 (femoral fracture)
|
9/26
|
|
2 K-wire
|
4
|
|
|
|
4/20
|
|
2 K-wire + TBW
|
4
|
1
|
|
1 (tibial fracture)
|
6/26
|
|
Minor complications
|
|
1 K-wire
|
|
3
|
1
|
|
4/28
|
|
1 K-wire + TBW
|
3
|
|
|
1 (wound dehiscence)
|
4/26
|
|
2 K-wire
|
|
|
2
|
|
2/20
|
|
2 K-wire + TBW
|
1
|
1
|
|
|
2/26
|
Abbreviations: SSI, surgical site infection; TBW, tension band wire.
a Implant-related complications included seroma, skin irritation, or any cause of implant
removal, for example, pin migration or patellar desmitis.
When the TTT was stabilized with a single K-wire, complications occurred in 9/28 cases,
with 6 of these requiring a second surgery either to remove the K-wire due to skin
irritation (n = 2), infection (n = 1) or pin migration (n = 2), or to treat a tibial tuberosity fracture (n = 1). There were four minor complications: three cases of patellar reluxation (grade
I) occurred but second surgery was not considered necessary, and one tibial tuberosity
fracture which was an incidental finding on follow-up radiographs and did not require
any specific treatment. One patient had two concurrent complications, with one being
classified as major (surgical site infection) and the other as minor (grade I patellar
reluxation).
When the TTT was stabilized with two K-wires, complications occurred in 6/20 cases.
The complications were classified as major in four cases and minor in two cases. Major
complications involved removal of the K-wires due to skin irritation (n = 2) or seroma formation (n = 2), and minor complications consisted of two tibial tuberosity fractures that did
not require further surgical or medical intervention.
When the TTT was stabilized with one K-wire and a TBW, complications occurred in 13/26
cases, with 9 of these requiring a second surgery. There were five implant removals,
one femoral growth plate fracture, one patellar reluxation and two tibial tuberosity
fractures. The implant removals were performed due to skin irritation (n = 3) and pin displacement (n = 2). Four minor complications occurred including seroma formation (n = 2), minimal wound dehiscence (n = 1) and minor open wound due to pin irritation (n = 1).
When the TTT was stabilized with two K-wires and TBW complications occurred in 8/26
cases, 6 of which were major and 2 were minor. Major complications included four implant
removals, one tibial fracture and one patellar reluxation (grade II). Implant removals
were caused by patellar ligament desmitis (n = 1), pin migration (n = 2) and seroma (n = 1); minor complications included seroma formation (n = 1) and patellar reluxation (n = 1, grade II).
Comparisons were made among the four construct groups (one pin, one pin-TBW, two pins,
two pins-TBW). There was no statistical difference in the incidence of complications
between the four different groups as separate categories (p = 0.850) or comparing them after grouping them by the use of one-two pins (p = 0.834), or by the use or not of a TBW (p = 0.560). There was no difference in the proportion.
Risk Factors Associated with Complications of Surgery for Patellar Luxation
Logistic regression analysis was used to determine factors associated with the occurrence
of complications, when taking into account possible confounding factors ([Table 2]). There were no significant interactions between TBW use and either bodyweight (p = 0.222) or whether or not the distal tibial tuberosity remained attached (p = 0.505). After the initial model was refined by backward-stepwise elimination, the
best-fit model was one that included four variables. In the final multiple regression
model ([Table 3]), the only factor positively associated with a reduced risk of complications was
preserving the TTT distal cortical attachment, whereas factors associated with an
increased risk of complications included medial release and caudoproximal pin direction.
Finally, these variables were further investigated to determine associations with
specific types of complication ([Table 4]). Medial release was associated with an increased risk of medial reluxation after
medial patellar luxation, while preserving the distal cortical attachment was associated
with a decreased risk of implant-related complications. Pin direction was not associated
with any specific type of complication.
Table 2
Simple logistic regression results determining factors associated with complications
after surgical correction of medial patellar luxation in dogs
|
Logistic regression
|
Complications
|
|
OR
|
95% CI[a]
|
p-Value
|
|
Age
|
0.99
|
0.98–1.01
|
0.966
|
|
Gender
|
1.46
|
0.63–3.40
|
0.374
|
|
Neuter status
|
2.50
|
1.03–6.05
|
0.042
|
|
Body weight
|
1.01
|
0.93–1.10
|
0.680
|
|
Surgeon's experience
|
0.73
|
0.30–1.77
|
0.493
|
|
Surgical time
|
0.99
|
0.98–1.01
|
0.794
|
|
Anaesthetic time
|
0.99
|
0.98–1.01
|
0.836
|
|
Trochleoplasty
|
0.31
|
0.08–1.21
|
0.092
|
|
TWR
|
1.27
|
0.52–3.13
|
0.590
|
|
TBR
|
0.33
|
0.10–1.09
|
0.069
|
|
Medial release
|
1.93
|
0.83–4.45
|
0.121
|
|
Lateral imbrication
|
0.59
|
0.20–1.67
|
0.321
|
|
Constructs
|
|
|
|
|
1 pin
|
0.83
|
0.33–2.11
|
0.708
|
|
2 pins
|
0.75
|
0.26–2.17
|
0.600
|
|
TBW
|
1.37
|
0.60–3.14
|
0.450
|
|
Distal tuberosity attachment
|
0.17
|
0.05–0.59
|
0.005
|
|
MPL grade
|
|
|
|
|
Grade I
|
Ref
|
|
|
|
Grade II
|
1.17
|
–
|
0.997
|
|
Grade III
|
1.30
|
–
|
0.997
|
|
Grade IV
|
1.17
|
–
|
0.997
|
|
K-wire insertion
|
|
|
|
|
Proximal to the tibial tuberosity
|
Ref
|
|
|
|
At the tibial tuberosity
|
0.45
|
0.16–1.20
|
0.113
|
|
Distal to the tibial tuberosity
|
0.94
|
0.27–3.22
|
0.928
|
|
K-wire direction
|
|
|
|
|
Caudodistal
|
Ref
|
|
|
|
Caudal
|
1.34
|
0.48–3.74
|
0.571
|
|
Caudoproximal
|
4.92
|
1.56–15.5
|
0.006
|
Abbreviations: CI, confidence interval; MPL, medial patellar luxation; OR, odds ratio;
Ref, reference; TBR, trochlear block recession; TBW, tension band wire; TWR, trochlear
wedge recession.
a 95% CI: ninety-five per cent confidence interval: Reference category used in logistic
regression. Variables highlighted in bold qualified for inclusion in the multiple
regression analysis at p < 0.20 ([Table 3]).
Table 3
Multiple logistic regression results determining factors associated with all complications
after surgical correction of medial patellar luxation in dogs
|
Logistic regression
|
Complications
|
|
OR
|
95% CI
|
p-Value
|
|
Distal tuberosity attachment
|
0.18
|
0.04–0.72
|
0.015
|
|
Medial release
|
3.22
|
1.00–10.34
|
0.048
|
|
Pin direction (caudoproximal)
|
4.53
|
1.01–20.36
|
0.048
|
Abbreviations: OR, odds ratio; 95% CI, ninety-five per cent confidence.
Table 4
Multiple logistic regression results determining factors associated with specific
types of complication after surgical correction of medial patellar luxation in dogs
|
Logistic regression
|
Complications
|
|
OR
|
95% CI
|
p-Value
|
|
Implant-related
|
|
|
|
|
Distal tuberosity attachment
|
0.10
|
0.01–0.82
|
0.032
|
|
Medial release
|
2.06
|
0.55–8.10
|
0.297
|
|
Pin direction
|
2.54
|
0.52–12.34
|
0.245
|
|
Reluxation
|
|
|
|
|
Distal tuberosity attachment
|
0.36
|
0.02–5.71
|
0.475
|
|
Medial release
|
9.94
|
1.14–86.05
|
0.037
|
|
Pin direction
|
2.26
|
0.14–34.41
|
0.555
|
|
TTF/avulsion
|
|
|
|
|
Distal tuberosity attachment
|
0.50
|
0.04–5.64
|
0.582
|
|
Medial release
|
1.40
|
0.17–11.24
|
0.745
|
|
Pin direction
|
1.62
|
0.12–20.49
|
0.709
|
Abbreviations: OR, odds ratio; 95% CI, ninety-five per cent confidence; TTF/avulsion:
tibial tuberosity fracture avulsion.
Discussion
This study has demonstrated that the constructs used to stabilize the TTT (1 pin,
2 pins, 1 pin with TBW, 2 pins with TBW) and the type of trochleoplasty (wedge vs
block) do not affect the occurrence of postoperative complications following medial
patellar luxation surgery in dogs weighing < 20 kg. Among the variables analysed,
preservation of the distal TTT cortical attachment was associated with a reduction
in the postsurgical complication rate, whereas performing a medial soft tissue release
was associated with an increase in the postsurgical complication rate.
The 37% overall complication rate reported in this study is higher than recently reported
but consistent with historical data.[8]
[9]
[10]
[11]
[12]
[13] However, results from different studies should be interpreted cautiously, as there
are important differences between them regarding case selection, length of follow-up,
surgical techniques, categorization of minor and major complications, presence of
concurrent orthopaedic pathology and comparison between unilateral, bilateral staged
and bilateral single session surgical procedure. Implant-related complications requiring
a second surgery represented the most frequent complication in our study (n = 17). The reason for this is unknown but the authors speculate that this might be
due to the relatively small soft tissue coverage around the surgical site which could
potentially increase the risk of skin irritation or seroma formation. Another explanation
is that in some cases an ongoing instability is likely to have contributed to pin
migration and, therefore, led to implant removal.
Patellar reluxation is a commonly reported complication and this finding is supported
by our study (n = 6). Reasons for patella reluxation can include an inadequate degree of TTT, concurrent
femoral or tibial angular limb deformities or a postsurgical inadequate trochlear
groove depth. It has been suggested that failure to address a shallow trochlear groove
is a risk factor for reluxation,[18] while dogs that undergo medial patellar luxation surgery without trochleoplasty
are five times more likely to develop reluxation.[9] A trochleoplasty was performed in 90% of our cases. The type of trochleoplasty performed
was decided on individual basis by the surgeon. In a cadaveric study comparing both
types of trochlear recession, trochlear block recession provided a larger recessed
percentage of trochlear surface area with increased patellar articular contact and
greater resistance to patellar luxation in a stifle extended position than trochlear
wedge recession.[19] It seems then plausible that patients that have a trochlear wedge recession would
have a higher postsurgical reluxation rate. Results from our study showed no significant
difference in complication rates between the two techniques. To the authors knowledge,
this is the first study comparing clinical outcomes of these two techniques.
Another major complication reported in the current study involved either fracture
or avulsion of the tibial tuberosity.[6] This was observed in 6% stifles, with three cases requiring additional surgery and
three cases being managed conservatively. Tibial tuberosity avulsions have previously
been associated with the use of a single pin to stabilize the TTT, and with a caudodistal
pin insertion angle.[6] It has also been suggested that the use of a TBW could prevent a tibial tuberosity
fracture.[19] However, our results contradict this in that we did not identify a specific construct
for stabilizing the tibial tuberosity associated with an increased risk of postsurgical
tibial tuberosity fracture or avulsions. An attempt was made to analyse if the location
of pin insertion in relation to the insertion of the patellar ligament would influence
the incidence of tibial tuberosity fractures or avulsion by acting as a stress riser.
Our results showed that placement of the most proximal pin proximal, distal or at
the level of insertion of the patellar ligament did not have any influence in the
rate of surgical complications.
Current recommendations on how to stabilize the TTT involve the placement of 1 or
2 K-wires, directed in a disto-caudomedial direction and the placement of a TBW.[20] Although it has been suggested that using a TBW minimizes the risk of implant-related
complications (migration or implant failure) and prevent tibial tuberosity fracture,[19] only about half of our patients had a TBW placed. No significant difference between
the four constructs and the occurrence of complication was found, as well as when
the three categories of complication rates (risk of tibial tuberosity fracture or
avulsions, risk of reluxation and risk of having an implant-associated complications)
were assessed. The only risk factors identified that were associated with having a
complication were lack of tibial tuberosity distal cortical attachment during TTT,
placing the pin in a caudoproximal direction and performing a medial retinacular release.
In the current study, the medial retinacular release was associated with increased
odds of any complication, especially having postoperative patellar reluxation. A possible
explanation is that medial release is more likely to be performed in higher grades
of medial patellar luxation in which it is challenging to reduce the patella into
the trochlear grove without this medial release, hence the negative association with
complications. Surprisingly, no significant association was found between the preoperative
medial patellar luxation grade and any specific type of complication; this might be
due to a type II statistical error. Another possibility could be that the quadriceps
mechanism would have not been adequately aligned due to inadequate TTT or failure
to recognize and correct femoral varus and therefore complication rates would be expected
to be higher. Due to the retrospective nature of the study, it was not possible to
assess if the degree of lateral TTT was adequate. However, it must be assumed that
the surgeon performing the procedure considered the degree of TTT lateralization adequate
during the procedure. Regarding the possible presence of a significant femoral varus,
preoperative radiographs and/or computed tomographic scans were not consistent with
this possibility.
Placing the pin in a caudoproximal direction was associated with increased overall
odds of developing a complication. Given the wide-ranging complications observed,
the fact that no association was observed with a specific type of complication on
multiple regression might be due to a type II statistical error. Therefore, further
studies could be considered to explore the complications associated with caudoproximal
pin placement.
The findings of the current study also suggest an association between tibial tuberosity
distal cortical attachment preservation and reduced odds of complications with medial
patellar luxation surgery, and in particular with a reduced risk of implant-related
complications. This suggests a possible increased tibial tuberosity instability when
the distal attachment is not preserved. Indeed, a previous study suggested that preservation
of the distal cortical extension of the osteotomy segment allows a partial transmission
of load forces to the tibia rather than relying solely on the implant fixation and
moreover, it is also important for preservation of blood supply.[21]
[22] Unfortunately in some postoperative radiographs, it was not possible to determine
if the distal TTT cortical attachment was intact, and therefore it is possible that
the lack of difference between constructs could have been due to an statistical error.
Only patients weighing < 20 kg were included in this study as body weight has been
reported to be risk factor for the development of complications with a cut-off of
20 kg: dog weighting > 20 kg had a higher frequency of overall and major complications
and reluxation compared with dogs weighing < 20 kg.[10]
Limitations of this study include its retrospective design, which relied on the use
of clinical records. In particular for techniques such as lateral imbrication and
medial soft tissue release, many methods and procedure exist. Based on the retrospective
nature of the study, it was not possible to determine what particular technique was
used in each dog. The measures of clinical outcome of the dogs in this study were
not evaluated. Subjective owner assessment using a questionnaire or kinematic analysis
would have been helpful to evaluate the functional outcome; however, this was beyond
the scope of this study.
We identified long-term complications by reviewing the referring veterinarian clinical
records; therefore, it is possible that some complications were not noticed by the
referring veterinarian and were missed. Third, there was variability in the corrective
procedures used, which was based on the preference of the surgeon, and there was no
standardized protocol to compare the four groups. It was not possible to quantify
if the TTT performed was sufficient to realign the quadriceps mechanisms or if the
trochleoplasty was deep enough to accommodate permanently the patella. Fourth, although
the patients were followed for a minimum of 12 months after surgery, it is possible
that later-onset complications might be missed. A fifth limitation is study power;
although our power analysis suggested that the study was adequately powered for some
variables, we cannot guarantee that this study was adequately powered for every single
variable investigated.
In conclusion, the results of the current study suggested no differences in complication
rates when different implant constructs for TTT stabilization are used during medial
patellar luxation surgery in dogs weighing < 20 kg. However, preserving the distal
cortical TTT attachment was associated with decreased postsurgical complications rate,
while adding a medial capsule or retinaculum release was associated with increased
post-surgical complications. Finally, the type of trochleoplasty performed (trochlear
block recession vs. trochlear wedge recession) was not associated with the occurrence
of postoperative complications.
