public class Joints_
{
    public Dictionary<string, int> joints_;
    public Joints_()
    {
        joints_ = new Dictionary<string, int>();
    }
    public Joints_(bool inizializza)
    {
        joints_ = new Dictionary<string, int>();
        for (int i=0; i<3; i++)
        {
            if (i == 0)
            {
                joints_["x"] = 678;
            }
            if (i == 1)
            {
                joints_["y"] = 2646;
            }
            if (i == 2)
            {
                joints_["z"] = 1234;
            }
        }
    }
    public void SaveWithJson(string fileName, ref Joints_ c)
    {
        File.WriteAllText(fileName, JsonConvert.SerializeObject(c, Formatting.Indented));
    }
}

public class Persona_
{
    public Dictionary<string, Joints_> persona_;
    public Persona_()
    {
        persona_ = new Dictionary<string, Joints_>();
    }
    public Persona_(bool inizializza)
    {
        persona_ = new Dictionary<string, Joints_>();
        for (int i = 1; i < 18; i++)
        {
            persona_["joint_" + i] = new Joints_(true);
        }
    }
    public void SaveWithJson(string fileName, ref Persona_ c)
    {
        File.WriteAllText(fileName, JsonConvert.SerializeObject(c, Formatting.Indented));
    }
}

{
"persona_": {
  "joint_1": {
    "joints_": {
      "x": 678,
      "y": 2646,
      "z": 1234
    }
   },
etc etc....

{
  "persona_": {
    "joint_1": {
        "x": 678,
        "y": 2646,
        "z": 1234
    },
etc etc....