3. Discussion
This case report proves unicondylar prosthesis remains intact
even after two subsequent periprostatic fractures, and fracture healing
can be achieved without complications. The clinical outcome was
excellent, and the patient regained almost full range of motion. To the
best of our knowledge, no cases have been reported in the literature
regarding such a fracture.
The goal of managing these injuries includes restoration of axial
alignment and length and stability to allow early mobilization. However,
achieving this goal remains to be challenging for orthopedic surgeons.
Because of poor bone quality and fracture location, nonunion is not
uncommon with periprosthetic distal femoral fractures after total knee
replacement, and nonunion rates have been reported between 0% to 50%
[11]. Preservation of soft tissue and osseous vascularity have been
recommended as much as possible to reduce nonunion rates [12]. We
think that the same principles should be applied in the surgical
management of periprosthetic fractures after UKR. Restoring axial
alignment was especially crucial in our case because we think
unicondylar prosthesis would less tolerate shear forces than TKR caused
by malalignment.
Fracture localization and displacement are essantial
considerations in the management of supracondylar periprosthetic femur
fractures. Despite the fact that the use of locked plating and
retrograde intramedullary nailing (IMN) has been recommended for
displaced supracondylar femoral fractures after TKR, no consensus exists
regarding the ideal treatment strategy. We think both strategies are
applicable for periprosthetic femoral fractures after UKR. Retrograde
IMN offers more stability in the presence of comminution of the medial
cortex but is limited by poor cancellous bone, type of prosthesis, and
pre-existing hardware in the proximal femur [13,14]. Due to the
inherent advantage of UKR design that the femoral notch is not occupied
by the femoral component, IMN may be more applicable. Locked plating has
been recommended for low supracondylar fractures due to difficulties in
achieving distal fixation with IMN and controlling varus collapse
[15]. In a recent meta-analysis, Li et al. compared the clinical
results of locked plates and retrograde IMN and found no statistically
significant difference in terms of union, operating time, and rates of
complication [16].
Several factors can be attributed to the fractures in our
patient. As we discussed previously, the replacement of an
intramedullary rod during the procedure, may have created a stress riser
effect, which eventually diminished the structural strength of the femur
[7]. Subsequent fractures with minor trauma may indicate that
osteoporosis might have played a role in the etiology. However, our
patient did not have any known patient-related risk factors or previous
fractures related to osteoporosis.
Meniscal bearing dislocation is a well-known complication of
UKR with mobile bearing insert due to inappropriate ligament balance and
component replacement [16]. Despite the fact that the patient’s knee
was exposed to shear forces during trauma, the insert remained in its
place. We think mobile-bearing causes more compressive and less tensile
and shear force on the implants, thus avoiding an excessive load at the
bone-implant interfaces. Thanks to this mechanism, components may not be
loose despite the load transferred during the trauma.
Although many systems are available for classifying
periprosthetic fractures after TKR, there is no such classification
regarding UKR periprosthetic fractures [17]. The reason for not
needing such a classification may be due to the relatively low incidence
of periprosthetic fractures after UKR than TKR. However, UCR can be
considered as the most applicable and inclusive classification for
periprostatic fracture after UKR [10]. This system is based on the
Vancouver classification and has been defined as a relatively simple
alternative that can be used to describe any periprosthetic fracture
[10,18]. According to UCR classification, our case had Type C
fracture, in which the fracture line was distant to the bed of the
implant. It was suggested that these fractures could be managed open or
closed osteosynthesis without involving the implant as we did in our
case.
In conclusion, despite subsequent fractures, if UKR is properly
replaced and has appropriate ligament balance, insert dislocation would
not occur, and the components would remain intact. In addition, bone
healing could be achieved with an excellent clinical outcome. The
treatment goal should be the restoration of alignment and achieving
stability to allow early mobilization.