Actually, tendinitis and tenosynovitis are essentially two aspects of the same condition. The term tendinitis usually refers to the part where the muscle transitions into the tendon, whereas tenosynovitis generally refers to the fibrous sheath over the tendon surface. Thus, tendinitis and tenosynovitis often coexist; tendinitis can lead to tenosynovitis, and conversely, tenosynovitis can also affect tendinitis, causing or exacerbating it.

Therefore, in clinical practice, there usually is not a clear distinction between tendinitis and tenosynovitis. It is common during surgeries for stenosing tenosynovitis to observe that the tendon is also congested and swollen, and even adhered to the tendon sheath. Thus, tendinitis and tenosynovitis are generally considered the same disease, merely referred to by two different names. However, if strictly distinguished, there is a certain difference, which lies in the location: tendinitis refers to the tendon itself, while tenosynovitis refers to the sheath outside the tendon.