Change in platelet counts was a common phenomenon after major joint surgery, and elevated PLTC occurred in about half of the patients who were admitted to the rehabilitation centre. Thrombocytosis was not an uncommon event, since 20.1% of the sample had a PLTC above 500 × 1009/L, but this condition did not affect the functional outcome. Despite the risk of thromboembolism being a great concern in patients with thrombocytosis, no subject with elevated PLTC developed lower limb DVT.
Some of the most common causes of secondary thrombocytosis include soft-tissue, pulmonary and gastro-intestinal infections, as well as tissue damage and malignancy [
1]. Furthermore, thrombocytosis is a common finding among patients with trauma, injuries and surgical interventions admitted to the ICU [
6]. The significance of elevated PLTC in critically ill patients has not been clearly elucidated, although this sequela has generally been considered a benign finding. Only one study has been published reporting elevated PLTC or thrombocytosis after major orthopaedic surgery [
10]. Surprisingly 163 (79.1%) of 206 subjects had elevated PLTC, with values from 400 to 450 × 100
9/L. However, the authors focussed on extreme thrombocytosis, ascribing this finding to alcohol abuse and infections, and they were doubtful whether the finding was unique to orthopaedic surgery. Among secondary thrombocytosis, surgery including coronary artery by-pass, major abdominal surgery and especially splenectomy [
3,
1] are fairly common
. Orthopaedic surgery was also reported as being associated with secondary thrombocytosis, but the finding concerned retrospective studies and thrombocytosis was generally attributed to other causes such as trauma and tissue damage [
1]. To our knowledge, this is the first prospective study with a homogenous orthopaedic surgical sample and it demonstrates that high PLTCs occur in 42.4% of patients after major joint surgery. The role and significance elevated platelets might play in this clinical condition is unclear, though several hypotheses can be suggested. The major functions of platelets are to prevent acute blood loss and to repair vascular walls and adjacent tissues after a local injury. To accomplish this, platelets secrete many mediators and cytokines that stimulate tissue regeneration by cell proliferation, cell migration and angiogenesis [
18]. It has been reported that more than 800 different proteins are released that act on different cell types, including osteoblasts [
19], fibroblasts, chondrocytes, myocytes, and tendon cells [
20-
22]. Therefore, it could be postulated that major orthopaedic interventions activate physiological processes that heal the surgical wound, similar to the processes occurring during local injury. An increase in the amount of platelets may represent a component of that complex network of cells and inflammatory mediators that are activated following injury in order to repair and regenerate the damaged tissue. The observed increase in ESR, fibrinogen and D-dimer levels could also be considered as a further expression of an inflammatory response to repair the surgical wound. In the present study, elevated PLTC and inflammatory products were detected at about 7–12 days after surgery, when the patients were admitted to the rehabilitation unit. A similar finding was reported by Ziaja et al., who detected thrombocytosis two weeks after surgical intervention [
11]. Likewise, in a study by Bunting et al., the highest PLTC occurred from nine to 22 days (mean 14 ± 3.0) after surgery [
10]. Since platelets secrete many mediators and cytokines for at least seven days after a local injury [
18], this time period could represent the peak of an adaptive response. In order to evaluate a potential age effect on platelet change, the enrolled subjects were divided in two groups according to an age cut-off of 75 years. Young people had significantly higher platelet counts than old people, at all time points. This finding could be explained by a better or more evident reactive-reparative response to the injury in young people compared with old people. It is also possible to consider that platelet change in old people is simply due to the progressive physiological multi-organ decline which occurs during ageing. A recent survey of PLTC in a large national cohort population found that the number of platelets decreases quickly in childhood, stabilises in adulthood, and further decreases with age [
23]. The risk of thromboembolic events remains the greatest concern in patients with thrombocytosis, and subjects with elevated PLTC could be more likely to develop venous thromboembolism occurrence after major joint surgery. However, the relationship between elevated PLTC and these adverse events has not been established. One study by Salim et al. reported that there was a trend toward increased DVT, and a significantly higher rate of pulmonary embolism among patients developing thrombocytosis after severe trauma. However, this was not found in similar studies [
3,
6]. Despite this concern, no subject with thrombocytosis developed DVT in the present study. Global functional improvement was detected after rehabilitation treatment, but thrombocytosis did not affect the functional outcome. Similarly, a previous study reported that mortality risk was not correlated with PLTC in hip fracture patients [
24]. On the other hand, univariate analysis of variables including age and blood parameters showed that these factors have an important role in functional recovery. Age was negatively correlated with FIM and BS both at admission and at discharge. Furthermore, BS and FIM were positively correlated with haemoglobin and albumin level. Similar findings have been reported in previous studies addressing the relationship between blood parameters, functional outcomes and mortality risk [
12,
25,
26] in subjects undergoing joint surgery. The findings indicate that patients admitted to rehabilitation after major joint surgery have the potential for improvement in motor function and global functionality, but those with lower haemoglobin and albumin level and those who are older may expect longer hospital stays and poorer functional outcomes. Indeed, older people stayed in hospital longer and had significantly poorer outcomes than young people according to all of the functional assessment measures employed. Furthermore, significantly better gait recovery was observed in young compared with old people.