Introduction
Achondroplasia is a common form of skeletal dysplasia which has autosomal dominant or sporadic inheritance.1 Fibroblast growth factor receptor 3 (FGFR3) mutation found at p16.3 locus of chromosome 4 leads to inhibition of proliferation and terminal differentiation of chondrocytes affecting sub-chondral growth. FGFR3 dimers stabilized the mutation in the transmembrane domain preventing lysosomal degradation leading to continuous inhibition of chondrocyte proliferation. FGFR3 is critical for chondrogenesis and osteogenesis and gain of function mutation in Achondroplasia lead to decreased bone mass and increased risk for fracture. FGFR3 also plays a pivotal role in fracture healing inhibiting initiation of chondrogenesis and hypertrophic differentiation of chondrocytes in soft callus.2
Anatomic variations include articular and peri-articular deformities which amplify the complexity of fracture fixation. Concomitant cardio-vascular, respiratory and neurological co-morbidities pose an additional risk during peri and post-operative period.
The estimated prevalence is 1:25,000 in general population but only few case reports are available in literature that deal with orthopaedic fractures and problems associated with implant choice, internal fixation and healing.
Achondroplastic bone has a defect in enchondral ossification which affects the proliferative zone of the physis producing less chondrocytes for ossification. Proliferative zone stacks chondrocytes contributing to the longitudinal growth. Fracture healing consist of haematoma formation, soft callus, hard callus and re-modelling. Fracture healing occurs through intra-membranous or enchondral ossification. Rigid fixation of fracture heals with intra-membranous ossification and relative stability heals with callus by enchondral ossification. FGFR3 mutation has also showed delayed healing in rigid fixation.
We present a case of fracture neck of femur in an achondroplasia patient and its management. Achondroplastic patients can pose various medical and technical challenges and paucity of literature needs high degree of planning when faced with similar cases. 3
Case Report
38 year old male following fall of sintex tank over the left hip presented with fracture neck of femur in severe coxa-vara (Figure 1) Pre-operative planning was done and primary fracture fixation was decided upon. Prosthetic replacement was deferred because of young age and absent arthritic features. Implant choice was cannulated cancellous screw. Sliding hip screw system was deferred because of the abnormal geometry of the proximal femur. Patient had no co-morbidities. Closed reduction with two 16mm partially threaded cannulated cancellous screw with washer was done. The screws were placed perpendicular to the fracture site achieving absolute compression. Third screw was attempted but couldn’t be placed because of the short and deformed neck (Figure 2).
Post-operatively patient was placed in non-weight bearing. 3 month follow up showed intact screws at compression site (Figure 3). At six month follow up, mild lysis was seen around the screw zone (Figure 4) and at 1 year follow up, screws started loosening (Figure 5). Implant removal was done. Now patient is at 18 month follow up, with neck of femur non- union (Figure 6). He is walking full weight bearing with walker with mild disability. The future course of events to be planned weather excision arthroplasty or custom made prosthesis to fit for the native femoral anatomy.
Figure 1
Plain radiograph showing left fracture neck of femur in anachondroplasia patient with severe coxa- vara.
Figure 2
Intra-operative C-ARM pictures showing fixation of Neck with cannulated cancellous screws achieving compression at fracture site
Figure 3
Plain radiograph with operated fracture neck of femur, compression maintained with cannulated cancellous screw at 3 month follow up with no loosening.
Figure 4:
Figure 4
Plain radiograph of pelvis at six month with operated left neck of femur showing osteolysis at fracture site and screw loosening.
Discussion
Fracture neck of femur with bilateral severe coxa-vara of 90 degree poses a major technical challenge in the management. Achondroplasia is characterised by defects in en-chondral bone formation leading to retarded cartilage growth, short-limb stature, Flared metaphysis, femoral head and acetabular asymmetry, metaphyseal angulation of knee and proximal femur leading to bowed deformities.3, 4
Literature search of case reports on peri-articular fracture management in adult achondroplasia patients showed only one case report by murphy et al. In their case, they had fixed a distal femur with intra-articular extension (AO 33B2) fracture with only percutaneous screws since anatomical adult or paediatric distal femur locking plates couldn’t match the patient’s distal femur anatomy.5
In adults, fracture neck of femur is managed by percutaneous screw management, dynamic hip screw or prosthesis.6, 7, 8 Dynamic hip screw was deferred because the bowed anatomy of proximal femur. Prosthesis replacement was not considered since young age, absence of arthritic features and altered geometry of the proximal femur for stem fixation.9, 10
Healing in fracture neck of femur fracture occurs through compression at the fracture site. Cancellous bone possess much larger internal contact surface area with abundant blood supply with increased mesenchymal stem cells for differentiation and healing by intra-membranous ossification.
Early failure of screw fixation in our case was because of osteopenia, defective fracture healing due to defective chondral transformation to bone. Placement of cannulated cancellous screws perpendicular to the fracture site achieving compression but high pauwel’s angle >90 deg led to high strain over the screws due abnormal bio-mechanics leading to screw loosening and failure of union at the fracture site. 8
Conclusion
Achondroplastic dwarfs generally live a normal life. They are also prone to peri-articular fractures because of anatomical variations in their bone. They might have un-diagnosed cardio-vascular and pulmonary co-morbidies which increase peri-operative risk.
We wanted to highlight the difficulties and importance of pre-op planning and implant choice of fracture fixation in an adult achondroplasia patient.