膝頭下少少有舊骨凸起

首先估計你年令應該在十多歲之間吧,
你這種情况很多這年紀的青年人,尤其以喜歡打籃球,排球,踢足球的人都會發生,
但你別太驚恐,這只是發育時期,由於過度肌肉的活動,牽拉到骨的發育中心,而引起了我們骨傷科一種叫骨骺炎的一種症狀,
若患者當時找到適當治療,令炎症消失後,才將運動量慢慢加大,在運動時戴上適合的保護墊,
問題就慢慢消失,
有些年青人出現症狀後,未有妥善處理,年令漸長後,症狀亦可能隨年令消失,但可能會遺留了在患處腫起,永遠的腫起來.但亦只會腫起,應該無其它後遺症的,請放心.....

Suggest you to see a private orthopaedic doctor first. If simple, may cause few hundreds dollar only but you got the diagnosis.
If the diagnosis is not simple and treatment cost a lot, tell him to refer you to public orthopaedic doctor.

I think this will be good for your case.
20s years old is very, very unlikely to be arthritsis as mentioned in the other answer.

If you do not know which doctor to see. You can send letter to me (As I cannot tell U here as It is 廣告宣傳)

Best wishes!

你很可能生骨刺，嚴重的話要做手術
可看骨科醫生，他會要求你照X光
如看政府醫生就要先排普通科門診寫轉介信
如看中醫就請看跌打

你好可能生骨刺，睇普通科醫生先，再由佢轉介比專科。