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Unicameral bone cyst (proximal femur) – curettage , bone grafting and plating

    Overview

    Learn the Unicameral bone cyst (proximal femur) : curettage , bone grafting and plating surgical technique with step by step instructions on OrthOracle. Our e-learning platform contains high resolution images and a certified CME of the Unicameral bone cyst (proximal femur) : curettage , bone grafting and plating surgical procedure.
    Simple or unicameral bone cysts are common benign lesions found in children and adolescents and were first described by Virchow in 1891. It is important to distinguish them from more aggressive lesions such as aneurysmal bone cysts. Simple bone cysts (SBC) will frequently heal spontaneously once the adjacent physis has closed at skeletal maturity but lesions are still observed in some adults.
    The majority of lesions are found in the metaphysis of long bones, the humerus being the most frequent site. However, any bone can be effected including the axial skeleton. Boys are more frequently affected than girls.
    Simple bone cysts (SBC) are frequently asymptomatic unless they result in pathological fracture. The effects of such fractures is largely determined by their location, one of the most significant being around the hip where such injuries can result in malunion and damage to the blood supply to the femoral head unless appropriately managed.
    Simple bone cysts can be treated by several modalities. More aggressive management is generally indicated for the proximal femur because of the consequences of femoral neck fracture.


    Indications

    INDICATIONS
    The indication for curettage, bone grafting and internal osteosynthesis depends on the location of the bone cyst. In the proximal femur the risks of fracture are significant. Malunion will often be into varus increasing the risk of subsequent fracture. In addition displaced fractures run the risk of avascular necrosis due to damage to the reticular vessels. In this instance a more aggressive form of management is justified and curettage with morcellised allograft and plate/screw internal stabilisation would be standard first line treatment in children more than 2-3 years old.
    SYMPTOMS & EXAMINATION
    Simple bone cysts are asymptomatic unless they result in pathological fracture.
    IMAGING
    Most SBC are identified on plain X ray. The lesions are typically in the metaphysis but can migrate into the diaphysis. They are generally central and will result in expansion of the bone cortex but typically no wider than the adjacent physis. There will be an intact cortical rim although this will often be very thin. Cysts that abut the physis are said to be active and those more than 1cm from the physis latent.
    The ‘fallen leaf’ sign is pathognomonic. It represents a small flake of cortical bone. MRI shows a fluid filled lesion.
    Simple bone cyst needs to be distinguished from aneurysmal bone cyst (ABC). The latter is more aggressive and tends to extend into the surrounding soft tissues. An ABC may be a primary neoplasm but in 30% of cases arises in a pre existing lesion such as osteoblastoma, fibrous dysplasia, chondroblatoma. The most important differential diagnosis is a Teliangectatic osteosarcoma so if ABC is suspected biopsy is mandatory.
    ABC are typically multiseptated and on MRI multiple fluid/fluid levels will often be visible.
    ALTERNATIVE OPERATIVE TREATMENT
    Several surgical options exist for treating SBC. Injection of steroid reduces fluid production by the cyst wall. Bone marrow injection accelerates consolidation/filling in of the cyst. Injection treatments have success rates between 60-80%.
    In the humerus, where the consequences and risk of re-fracture are less, there is a tendency to be less aggressive although flexible intra-medullary nails have shown rates of consolidation and union above 95%.
    Autograft has not been shown to be more effective than allograft as a space occupier. However, in recurrent cases structural autograft such as fibula may be a viable option. Against this one has to consider potential morbidity at the donor site.
    NON-OPERATIVE MANAGEMENT
    Non operative management is acceptable treatment for SBC especially in small children or for fractures in non weight bearing bones. Approximately 15-20% of lesions will fill spontaneously following fracture.
    CONTRAINDICATIONS
    Proximal femoral fractures in very young children may be too small to support metalwork. In this instance hip spica treatment is a viable option.

    Setup

    Surgery is performed under general anaesthesia. If epidural anaesthetic is used for analgesic purposes a urinary catheter will be necessary.

    Operation – 1

    A coronal MRI showing the dramatic appearance of a large right side unicameral bone cyst in an adolescent patient.

    The patient is in lateral position throughout the operation with the operative limb uppermost. A pillow is placed between the legs. A spinal support is utilised and a small anterior post may be useful in the larger patient. However, props must be proximal enough that and image intensifier can have access for imaging of the entire proximal femur (see set up for proximal femoral varus osteotomy). The props do not need to hold the patient completely rigid and a degree of ‘looseness’ makes imaging easier per operatively. The posterior support should be over the lumber spine. Anterior support may not be needed in smaller children or if it is, then placed at level of abdomen (with plenty of padding and not tight).

    A direct lateral approach is utilised. The greater trochanter is palpated and marked out. The incision runs distally from this point. The length of incision will be determined by the size of the cyst but bear in mind that metalwork will have to extend for several cm distal to this.

    Sharp dissection extends through skin and fat. The tensor fascia lata is cleared and incised longitudinally. The vastus lateralis is clearly visualised. This should ideally be reflected forwards. An inverted ‘L’ incision is used starting at the proximal attachment of vastus lateralis into the greater trochanter. The leg is internally rotated by the assistant to enable visualisation of the origin of vastus into the femur posteriorly. Leave a small cuf of muscle (0.5cm) attached to the bone and work proximal to distal with diathermy taking care to coagulate perforating vessels (1 or 2 will be running from posterior compartment forwards).

    The periosteum is a relatively thick layer in children. It is split and elevated off the femur so that Homan type retractors can be placed around the bone. Care needs to be taken since the cortical shell may be extremely thin. Often it is so thin it is almost translucent, as in this case.

    To create access to the cyst a window is made in the lateral cortex. This is marked out by making multiple drill holes with 2mm drill and connecting with a small sharp osteotome.

    A sample of the cyst ling is taken using a curette. This is sent for histology.

    The contents of the cyst are generally serous or blood tinged fluid. This can be aspirated.

    It may be necessary to enlarge the window using a fine bone nibbler.

    The lining of the cyst needs to be carefully removed using a curette. Sometimes a small dental burr can also be useful.

    The image intensifier (II) is draped in a horizontal position so that the x ray beam is parallel to the ground. The II is then advanced from the bottom of the table. In this position, with the patient in a lateral position an AP image of the hip is obtained and by putting the leg in ‘frog lateral’ position one obtains a lateral view.

    Using image intensifier it is possible to verify that the whole extent of the lesion has been curreted.

    Studies would suggest no advantage of autograft over allograft. The defect needs to be packed tightly so it is important to have sufficient graft. One femoral head should be sufficient. It does not require thawing prier to milling.

    A bone mill provides good quantities of fine ground graft.

    Damp swabs are placed around the edges of the cortical window to prevent spillage into the soft tissues. A spoon or cobb elevator is useful for inserting the graft.

    The graft is packed as tightly as possible using a punch.

    The cortical window does weaken the bone despite the fact that graft has been inserted. Curretage and burring of the wall of the has to remove the lining as much as possible but this will also potentially weaken the femur and risk fracture. Ideally the cortical window should be placed low enough in the metaphysis such that it can be bridged by metalwork.

    In adults and adolescents who are nearing skeletal maturity inrtra medullary stabilisation is ideal and biomechanically the strongest form of fixation. It is absolutely contra indicated to use piriformis entry nails if the capital femoral physis is open due to the risk of causing avascular necrosis (damage to reticular vessels). Lateral entry femoral nails can be used in older children when the medullary canal is >7mm diameter. In younger children plate and screw fixation is preferred. Several companies manufacture suitable implants. It is imperative to choose an implant where locking screws can be passed up the femoral neck. In this instance we are utilising the Orthopaediatric proximal femoral locking plate and screw system. Place the implant on the lateral side of the femur such that it bridges the cortical window and insert drill ‘post’. Then obtain radiograph using image intensifier.

    With the patient in a lateral position an AP view of the hip is achieved using the image intensifier. The correct plate will allow 2 or 3 screws to be passed up the femoral neck engaging in the bone proximal to the physis. Two or 3 distal screws beyond the cyst are required in the diaphysis. One should aim to insert a good sized implant since the cyst may continue to grow over time necessitating revision. The orthopaediatric proximal locking plates come in 3.5mm and 4.5mm screw sizes and angles from 90 degrees to 150 degrees. In this instance a 140 degree 4.5mm plate was chosen.

    Once happy with the position of the plate a temporary guide wire can be passed through a small hole at the apex of the plate. Avoid gripping the plate with bone clamps as this risks fracturing the fragile bone.

    A drill is passed through the post and advanced under image intensifier control up to but not across the physis. After measuring the depth of the hole created a locking screw is inserted.

    The aim is to pass screws up the middle of the femoral neck. In younger children (pre teen) screws should not pass across the physis due to risk of growth arrest. If smooth wires are used the physis can be crossed and in older children (age >12 females, >14 males) screws can cross the physis since little growth remains. When the cyst is extensive there may be little space between the physis and the cyst. In order to gain maximum support and stability the screws should be as near the physis as possible without being across it.

    By flexing the hip and abducting the hip a frog lateral view can be obtained without having to move the image intensifier. It is critical to ensure that all screws are within the femoral neck.

    Once the proximal screws are inserted the distal stabilisation is completed by inserting 2 or 3 locking screws distal to the cyst.

    Unicaneral bone cysts can persist and enlarge despite all attempts at removal and filling. It is wise to get any implant/screws several centimetres beyond the cyst. ensure satisfactory placement using image intensifier.

    Operation – 2

    The vastus lateralis is replaced on top of the plate and repaired with absorbable 1/0 suture (vicryl).

    Tensor fascia Lata (TFL) is repaired with 1/0 absorbable suture (vicryl).

    Fat is closed with absorbable 2/0 suture (vicryl) and subcuticular suture (3/0 monocryl) to skin.

    Post operative radiograph shows obliteration of cyst with allograft. Satisfactory placement of metalwork spanning the area of weakness.

    Post Operative

    Providing rigid fixation has been obtained immobilisation with cast is not necessary. The patient should not weight bear for 4-6 weeks.
    In younger children who may have less rigid stabilisation (Titanium elastic intramedullary nails TENS, Nancy Nails) and who may be less reliable, a broomstick cast may be used and in children less than 5 years old a hip spica may be indicated.

    Recommended Reading

    The published results of treatment of unicameral bone cysts are quite heterogeneous. Many different techniques are utilised and treatment preferences differ depending on which bone is affected and on what age the patient. Because of the consequences of fracture around the proximal femur treatment does tend to be more aggressive and grafting and internal fixation are the most frequent approach. Autograft does not appear to offer advantages over allograft. Internal fixation does not speed healing but does speed rehabilitation since patients are safe to weight bear more quickly. Internal fixation also reduces risk of pathological fracture. Overall consolidation/healing rates for this technique are approximately 90%.
    The following references provide a good overall view:
    Orthopedics. 2017 Sep 1;40(5):e862-e867. doi: 10.3928/01477447-20170810-01. Epub 2017 Aug 18.
    Treatment of Unicameral Bone Cysts of the Proximal Femur With Internal Fixation Lessens the Risk of Additional Surgery. Wilke B, Houdek M, Rao RR, Caird MS, Larson AN, Milbrandt T
    Arch Orthop Trauma Surg. 2016 Aug;136(8):1051-61. doi: 10.1007/s00402-016-2486-9. Epub 2016 Jun 17. A treatment strategy for proximal femoral benign bone lesions in children and recommended surgical procedures: retrospective analysis of 62 patients. Erol B1, Topkar MO2, Aydemir AN1, Okay E1, Caliskan E1, Sofulu O1.
    Journal of Children’s Orthopaedics. 2014 Feb;8(2):171-191. Treatment of unicameral bone cyst: Systematic review and meta analysis. Kadhim M, Kadhim A, Thacker M, Holmes L.

    Reference

    • orthoracle.com

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