The use of injectable calcium phosphate
bone cement in tibial plateau fractures for structural support

HATZIANTONIOU J., DIAKOS G., KYRIAKIDIS M.
Orthopaedic Department, G.H. Mytilini

ABSTRACT
AIM of this paper is to show the results of the use of injectable calcium phosphate bone cement for the filling of bone defects in tibial plateau fractures that have been treated operatively.
Material: Twenty patients (9 female and 11 male), 19-76 years old (average age 47.2) that have been surgically treated for tibial plateau fractures and in which calcium phosphate has been used. According to the Schatzker classification four of those were type II fractures, twelve type III, three type IV and one type VI fracture. All patients have had a minimum of twelve months follow-up.
Method: Fifteen of the patients have been treated with plate and screws, two with a single screw, two with K-wires and one with the combination of an external fixator and a K-wire. The bone defect that was created after the reduction of the tibial condyles was filled with calcium phosphate bone cement. Full weight bearing was initiated two months post-op in type II and III fractures. In follow-up, the amount of articular surface depression was measured in comparison to the healthy side.
Results: In eighteen of the patients, no depression of the operated condyle, or loss of reduction was observed. In one patient that started full weight bearing one month post-op, there was depression of the condyle for 4 mm. In another patient with osteoporotic bone and type III fracture, and in which weight-bearing was initiated two months post-op, there was significant collapse of the lateral condyle.
Conclusion: Injectable calcium phosphate bone cement, used for structural support, in open reduction and internal fixation of tibial plateau fractures alternatively to bone graft, other than the theoretical advantages, is easier, faster and safer in application. Furthermore, we found that it allows earlier initiation of weight bearing, in comparison to bone graft, at least in the younger patients with good quality of bone.

Key words: Tibial plateau fracture, triphosphate calcium.

INTRODUCTION
Tibial plateau fractures are intraarticular fractures of the knee. They are usually treated surgically, because we aim to anatomic reduction the articular surface. This can be achieved either with open reduction, or recently by other methods such as reduction under arthroscopic control, or radiographic screening[12].
The usual intraoperative problem with these fractures has to do with the bone defect that is created in the tibial metaphysis after the reduction of the fractured condyle. This defect is caused by the compression of the relatively soft cancelous bone of the condyles that takes place both at the time of the original injury and the reduction procedure.
The filling of this defect can be achieved with various ways. Traditionally, this was done with autograft that was taken from the ilium. Allograft, xenograft and bone substitutes can also be used.
Lately, manufactured bone grafts and bone substitutes have become increasingly popular. The indications for their use however, have not been accurately determined. The few papers that have been published to date are based on research on animals, or have been sponsored by companies producing these grafts. The small number of relative clinical studies has lead us to conduct this study.
Auto-, xeno-, and allografts have the disadvantage of providing poor mechanical support until they are integrated in the host bone, procedure that might take longer than six months[17].
One school of thought suggests that acrylic bone cement could be used, especially in elderly patients with osteoporotic bone, so that early weight bearing can be initiated, as it has been recommended for intertrochanteric[14] and calcaneal[7] fractures. This method has the disadvantage of non integration, while the healing process can be compromised by the high temperature that is generated when the bone cement polymerization takes place. Furthermore, the bone healing is also jeopardized by the fact that bone cement does not allow contact between the bone fragments. The relatively new material that has been used in this study is the bone substitute Norian SRS. This is an injectable form of calcium phosphate that sets in approximately ten minutes after the insertion in the bone cavity, in environment temperature, offers significant initial mechanical support, while it does not extensively interact with the bone[1].

 

1A.

2A.
Figure 1. A. B. At the lateral x-ray, 5 days pop, a piece of calcium phosphate is seen intraarticullarly. At the 5th pop month, this piece was resorbed. In spite if this, 30 months pop, there was no radiological evidence of calcium phosphate resorption from the fracture site.
5 months post-surgery

AIM
Aim of this paper is to publish the results in using this form of bioactive calcium phosphate for filling bone defects in tibial plateau fractures.

MATERIAL
Twenty patients with tibial plateau fractures that were operated, in which calcium phosphate was used are included in this paper. These are nine female and eleven male with an age range of 17-76 years. All these patients have been followed up for more than twelve months. Those with less than twelve months follow-up have not been included in this study. A patient that sustained infection was also excluded from this study, because the mechanical properties of calcium phosphate are influenced by the change in pH that accompanies inflammation1. According to the Schatzker classification four of the fractures that are included in our study were type II fractures, twelve type III, three type IV and one type VI fracture (table 1).

2.
Figure 2. In this case, we can see radiologically, calcium phosphate resorption 15 months pop.

METHOD
Fifteen of the patients have been treated with plate and screws, two with a single screw, two with Kirschner wires and one that suffered from Osteogenesis Imperfecta with the combination of an external fixator and a Kirschner wire. A lateral approach was used in all cases except from the three type IV fractures. In those, we followed a straight skin incision and approach of the condyles through bilateral thylacotomy.
After the manipulation, the reduction was usually maintained with K-wires. Subsequently, the morphology of the bone defect was explored by the surgeon with a pair of curved forceps, so that the direction towards which the needle for the injection of the calcium phosphate should target would be decided. Afterwards, the internal fixation was applied (plate with screws, single screws, etc.), followed by the injection of the calcium phosphate bone cement, through the fracture itself, or through a hole that was drilled for this purpose. Wound closure started ten minutes after the injection, because this is when the injected material starts to set. Special care was taken that calcium phosphate was not injected intraarticularly. If this was the case, thorough washout took place, until no cement was left on the articular surfaces. Nevertheless, in one case, some intraarticular cement was detected radiographically, but this caused no clinical problems. A full cylinder Plaster of Paris including the foot was applied for at least 24 hours post-operatively, until the calcium phosphate bone cement reached its maximum strength.
In the immediate post-operative period, all the patients were initially mobilized non weight-bearing and then partial weight-bearing was introduced gradually, subject to the stability of the fixation. Patients with type II and III fractures and good quality of underlying bone walked full weight-bearing after two months post-operatively.

RESULTS
The depression of the operated condyle was measured in comparison to the healthy side, usually the medial condyle. The post-operative follow-up varied between twelve and thirty months.
No loss of reduction was detected in patients with good quality of bone and initiation of full weight-bearing after the second post-operative month.
In one case, in a 36 years old patient, with a type III fracture, who started full weight-bearing one month post-operatively against medical advice, 4mm of depression of the fractured condyle were measured. Nevertheless, the clinical result remains good 24 months after the operation, while no osteoarthritic changes have been identified radiologically to date.
In another patient, 73 years old, with osteoporosis, type III fracture, who started full weight-bearing after the second post-op month, significant collapse of the lateral tibial condyle was observed. In one patient, leakage of the calcium phosphate bone cement was found in the check x-ray. However, this caused no clinical problems, possibly because the material did not directly interfere with the articulation of the joint (picture 1). The calicium phosphate was absorbed from the intraarticular space six months post-operatively.
The resorption of the calcium phosphate material that was injected in the tibial condyles could not be predicted or preassessed. In some of the patients, gradual absorption was apparent in the radiographic follow-up, while in others, no indication of the material absorption was identified in the x-rays up to 2.5 years post-op.
Nevertheless, full resorption was not found in any of the patients.

DISCUSSION
The classic method for filling the bone defect that is created in the metaphysis after the reduction of the fractured tibial condyle, is the application of autograft, obtained from the ilium. This method however has significant disadvantages such as local complications: infection, haematoma, persisting post-op pain, neurological deficit, deformed scar, restriction in the amount of obtainable graft, delayed mobilization[3,13,20]. The graft does not completely fill the empty space in the metaphysis, and so rapid integration is compromised. Satisfactory integration of the graft into the bone takes at about 6-12 months. During this period, collapse of the articular surface can theoretically happen, if full weight-bearing was to be applied.
For these reasons, various different types of bone grafts and substitutes have been used. Calcium phosphate is substitute for bone graft. It is a material relatively similar to the inorganic part of bone tissue. Its endurance to axial forces is higher than the one of the cancellous metaphyseal bone. Cancellous bone can endure compression forces 2-20 Mpa, while calcium phosphate 55 Mpa1. It does not have high endurance to rotational forces. Furthermore, during the insertion of this material in the bone defect, it occupies the whole volume of the bone cavity, as it is in a liquid form.
As a result, when it sets, it offers better structural support to the fracture. So, it has theoretical advantages when used in filling the bone defects in cancellous bone fractures, such as in the calcaneus[15,16], the distal radius[8,9,15] and the proximal tibial metaphysis[5,6,11].
Yetkinler et al[19], induced compression tibial plateau fractures in cadaveric bone in which subsequently they applied either internal fixation or calcium phosphate. After that, they applied axial loads on their material and concluded that calcium phosphate can bear higher forces than internal fixation. The same conclusion was reached by Welch et al[18] in their study of laboratory animals. Trials in patients have not been published. Horstmann WG et al.[5] report loss of reduction in one of 14 patients with tibial plateau fractures that were treated with internal fixation and injectable calcium phosphate. Lobenhoffer et al[11] report two cases of loss of reduction out of a total of 26 cases. Keating JF et al.[6] only use AO screws and occasionally Kirschner wires for the internal fixation. All the patients were mobilized full-weight bearing by the 6th postoperative week. They report that eight out of their 44 patients sustained some loss of reduction. In seven, the depression was less than 3mm and the writers connect the loss of reduction to old age. In the eighth patient who had infection, the loss of reduction was expected, since Norian loses its endurance in pH changes.
In our study, loss of reduction was observed in two out of 20 patients. One of the two started full weight-bearing against the advice given to him on the fifth week post-operatively. In the second, 73 y.o. female, full weight bearing was introduced after the second month post-operatively, according to our advice. Her osteoporotic quality of bone has probably played a role in the loss of reduction, as it is also described in the literature[6]. The initiation of full weight-bearing, should therefore be more conservative in patients with poor quality of underlying bone.
Calcium Phosphate that has been used, as long as the rest bone substitutes, have osteoconductive properties[13]. They act as a frame in which new vessels are formed. These, carry primitive osteocytes which, when mature, produce osteoid tissue. The graft itself is then gradually resorbed by means of fagocytosis by the osteoclasts, as described in studies where laboratory animals were used[2,4]. Theoretically, the resorption of calcium phosphate takes place rapidly, since this is proportionate to the contact surface between graft and bone, which in our case is maximum. Schildauer et al[16] who reported the use of calcium phosphate in calcaneum fractures sampled biopsies from seven patients at the time of metalwork material removal at least one year after the injury. In those samples, progressed resorption of calcium phosphate was observed as long as replacement by normal osteoid matrix and absence of fibrous tissue.
However, the resorption of the material in this and other studies[5,6], cannot be predicted. There is little understanding regarding the reasons that result to the progressed resorption by the 15th post-operative month in some patients, while in others there is no indication of resorption even two years post-op. At this point, we should also mention that both our data and the rest of the literature are based on radiographic screening and not in histological evidence.
One disadvantage of the technique is the high cost of the material. If however we take into account the possible complications that might follow the procedure of producing autograft and their subsequent cost, in addition to the fact that calcium phosphate bone cement allows early weight-bearing, shorter rehabilitation period and earlier return to work, we may say that the high cost of the material is justified. Possibly, the overall cost might even be lesser than in other methods.

CONCLUSION
The use of injectable bioactive calcium phosphate bone cement in the treatment of tibial plateau fractures seems to minimize the period of partial loading, at least in younger patients, and it results to faster rehabilitation. It is easier to use by the Orthopaedic Surgeon, compared to autografts while a second procedure (for obtaining the autograft) along with its complications is avoided. The process of the material resorption and whether this is finally partial or complete needs to be studied further.

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